Grind out the basic unit values (null and undef).

Almost the same as the old ones.  Builders and Styles change, ofc.

Also took the liberty of adding TypeName values to all ErrWrongKind.
It's apparently now the convention that untyped things set that with
the lowercase kind name.

_rsrch/nodesolution/unit.go

+package ipld
+
+var Null Node = nullNode{}
+
+type nullNode struct{}
+
+func (nullNode) ReprKind() ReprKind {
+	return ReprKind_Null
+}
+func (nullNode) LookupString(key string) (Node, error) {
+	return nil, ErrWrongKind{TypeName: "null", MethodName: "LookupString", AppropriateKind: ReprKindSet_JustMap, ActualKind: ReprKind_Null}
+}
+func (nullNode) Lookup(key Node) (Node, error) {
+	return nil, ErrWrongKind{TypeName: "null", MethodName: "Lookup", AppropriateKind: ReprKindSet_JustMap, ActualKind: ReprKind_Null}
+}
+func (nullNode) LookupIndex(idx int) (Node, error) {
+	return nil, ErrWrongKind{TypeName: "null", MethodName: "LookupIndex", AppropriateKind: ReprKindSet_JustList, ActualKind: ReprKind_Null}
+}
+func (nullNode) LookupSegment(seg PathSegment) (Node, error) {
+	return nil, ErrWrongKind{TypeName: "null", MethodName: "LookupSegment", AppropriateKind: ReprKindSet_Recursive, ActualKind: ReprKind_Null}
+}
+func (nullNode) MapIterator() MapIterator {
+	return nil
+}
+func (nullNode) ListIterator() ListIterator {
+	return nil
+}
+func (nullNode) Length() int {
+	return -1
+}
+func (nullNode) IsUndefined() bool {
+	return false
+}
+func (nullNode) IsNull() bool {
+	return true
+}
+func (nullNode) AsBool() (bool, error) {
+	return false, ErrWrongKind{TypeName: "null", MethodName: "AsBool", AppropriateKind: ReprKindSet_JustBool, ActualKind: ReprKind_Null}
+}
+func (nullNode) AsInt() (int, error) {
+	return 0, ErrWrongKind{TypeName: "null", MethodName: "AsInt", AppropriateKind: ReprKindSet_JustInt, ActualKind: ReprKind_Null}
+}
+func (nullNode) AsFloat() (float64, error) {
+	return 0, ErrWrongKind{TypeName: "null", MethodName: "AsFloat", AppropriateKind: ReprKindSet_JustFloat, ActualKind: ReprKind_Null}
+}
+func (nullNode) AsString() (string, error) {
+	return "", ErrWrongKind{TypeName: "null", MethodName: "AsString", AppropriateKind: ReprKindSet_JustString, ActualKind: ReprKind_Null}
+}
+func (nullNode) AsBytes() ([]byte, error) {
+	return nil, ErrWrongKind{TypeName: "null", MethodName: "AsBytes", AppropriateKind: ReprKindSet_JustBytes, ActualKind: ReprKind_Null}
+}
+func (nullNode) AsLink() (Link, error) {
+	return nil, ErrWrongKind{TypeName: "null", MethodName: "AsLink", AppropriateKind: ReprKindSet_JustLink, ActualKind: ReprKind_Null}
+}
+func (nullNode) Style() NodeStyle {
+	return nullStyle{}
+}
+
+type nullStyle struct{}
+
+func (nullStyle) NewBuilder() NodeBuilder {
+	panic("cannot build null nodes") // TODO: okay, fine, we could grind out a simple closing of the loop here.
+}
+
+var Undef Node = undefNode{}
+
+type undefNode struct{}
+
+func (undefNode) ReprKind() ReprKind {
+	return ReprKind_Null
+}
+func (undefNode) LookupString(key string) (Node, error) {
+	return nil, ErrWrongKind{TypeName: "undef", MethodName: "LookupString", AppropriateKind: ReprKindSet_JustMap, ActualKind: ReprKind_Null}
+}
+func (undefNode) Lookup(key Node) (Node, error) {
+	return nil, ErrWrongKind{TypeName: "undef", MethodName: "Lookup", AppropriateKind: ReprKindSet_JustMap, ActualKind: ReprKind_Null}
+}
+func (undefNode) LookupIndex(idx int) (Node, error) {
+	return nil, ErrWrongKind{TypeName: "undef", MethodName: "LookupIndex", AppropriateKind: ReprKindSet_JustList, ActualKind: ReprKind_Null}
+}
+func (undefNode) LookupSegment(seg PathSegment) (Node, error) {
+	return nil, ErrWrongKind{TypeName: "undef", MethodName: "LookupSegment", AppropriateKind: ReprKindSet_Recursive, ActualKind: ReprKind_Null}
+}
+func (undefNode) MapIterator() MapIterator {
+	return nil
+}
+func (undefNode) ListIterator() ListIterator {
+	return nil
+}
+func (undefNode) Length() int {
+	return -1
+}
+func (undefNode) IsUndefined() bool {
+	return true
+}
+func (undefNode) IsNull() bool {
+	return false
+}
+func (undefNode) AsBool() (bool, error) {
+	return false, ErrWrongKind{TypeName: "undef", MethodName: "AsBool", AppropriateKind: ReprKindSet_JustBool, ActualKind: ReprKind_Null}
+}
+func (undefNode) AsInt() (int, error) {
+	return 0, ErrWrongKind{TypeName: "undef", MethodName: "AsInt", AppropriateKind: ReprKindSet_JustInt, ActualKind: ReprKind_Null}
+}
+func (undefNode) AsFloat() (float64, error) {
+	return 0, ErrWrongKind{TypeName: "undef", MethodName: "AsFloat", AppropriateKind: ReprKindSet_JustFloat, ActualKind: ReprKind_Null}
+}
+func (undefNode) AsString() (string, error) {
+	return "", ErrWrongKind{TypeName: "undef", MethodName: "AsString", AppropriateKind: ReprKindSet_JustString, ActualKind: ReprKind_Null}
+}
+func (undefNode) AsBytes() ([]byte, error) {
+	return nil, ErrWrongKind{TypeName: "undef", MethodName: "AsBytes", AppropriateKind: ReprKindSet_JustBytes, ActualKind: ReprKind_Null}
+}
+func (undefNode) AsLink() (Link, error) {
+	return nil, ErrWrongKind{TypeName: "undef", MethodName: "AsLink", AppropriateKind: ReprKindSet_JustLink, ActualKind: ReprKind_Null}
+}
+func (undefNode) Style() NodeStyle {
+	return undefStyle{}
+}
+
+type undefStyle struct{}
+
+func (undefStyle) NewBuilder() NodeBuilder {
+	panic("cannot build undef nodes") // this definitely stays true.
+}