Add a schema.Kind.ActsLike() ReprKind mapping.

I wanted this for use in codegen work: in particular, as part of
generating error messages.

Note the comment on the case for unions.  I still go back and forth on
this topic; see some recent previous commit messages on this branch.
(I think I'm slowing coming around to "unions act like maps", and it's
just that most library code chooses to jump over them transparently,
because that's the least-odd path for resolving NodeBuilder questions;
but it remains the case that none of this is settled for sure yet.)

schema/kind.go

 package schema
 
+import (
+	ipld "github.com/ipld/go-ipld-prime"
+)
+
 // Kind is an enum of kind in the IPLD Schema system.
 //
 // Note that schema.Kind is distinct from ipld.ReprKind!
@@ -19,8 +23,8 @@ const (
 	Kind_String  Kind = 's'
 	Kind_Bytes   Kind = 'x'
 	Kind_Link    Kind = '/'
-	Kind_Union   Kind = '^'
 	Kind_Struct  Kind = '$'
+	Kind_Union   Kind = '^'
 	Kind_Enum    Kind = '%'
 	// FUTURE: Kind_Any = '?'?
 )
@@ -47,13 +51,58 @@ func (k Kind) String() string {
 		return "Bytes"
 	case Kind_Link:
 		return "Link"
-	case Kind_Union:
-		return "Union"
 	case Kind_Struct:
 		return "Struct"
+	case Kind_Union:
+		return "Union"
 	case Kind_Enum:
 		return "Enum"
 	default:
 		panic("invalid enumeration value!")
 	}
 }
+
+// ActsLike returns a constant from the ipld.ReprKind enum describing what
+// this schema.Kind acts like at the Data Model layer.
+//
+// Things with similar names are generally conserved
+// (e.g. "map" acts like "map");
+// concepts added by the schema layer have to be mapped onto something
+// (e.g. "struct" acts like "map").
+//
+// Note that this mapping describes how a typed Node will *act*, programmatically;
+// it does not necessarily describe how it will be *serialized*
+// (for example, a struct will always act like a map, even if it has a tuple
+// representation strategy and thus becomes a list when serialized).
+func (k Kind) ActsLike() ipld.ReprKind {
+	switch k {
+	case Kind_Invalid:
+		return ipld.ReprKind_Invalid
+	case Kind_Map:
+		return ipld.ReprKind_Map
+	case Kind_List:
+		return ipld.ReprKind_List
+	case Kind_Unit:
+		return ipld.ReprKind_Bool // maps to 'true'.
+	case Kind_Bool:
+		return ipld.ReprKind_Bool
+	case Kind_Int:
+		return ipld.ReprKind_Int
+	case Kind_Float:
+		return ipld.ReprKind_Float
+	case Kind_String:
+		return ipld.ReprKind_String
+	case Kind_Bytes:
+		return ipld.ReprKind_Bytes
+	case Kind_Link:
+		return ipld.ReprKind_Link
+	case Kind_Struct:
+		return ipld.ReprKind_Map // clear enough: fields are keys.
+	case Kind_Union:
+		return ipld.ReprKind_Map // REVIEW: unions are tricky.
+	case Kind_Enum:
+		return ipld.ReprKind_String // 'AsString' is the one clear thing to define.
+	default:
+		panic("invalid enumeration value!")
+	}
+}