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This first crossed my mind when considering whether it might be a good
idea to export the concrete implementation types of the 'basicnode'
package (see previous couple of commits -- it mostly had to do with the
question of whether we could reuse more code and shrink the size of
packages emitted by codegen)... but even though that's resolved to a
pretty solid "nope", it still makes sense to have a consistent name
prefix pattern here... and we'll probably encourage and expect to see
similar conventions in other packages (including in codegen, if we
add a handy-constructor-funcs feature to that).

Turns out there was a lot of tech debt already accumulating very very
rapidly due to the earlier whimiscal choice to lump the new basicnode
implementations and the demo-codegen implementations into the same
packages as benchmarking and research on this branch began.

The basicnode package is now extracted cleanly, and exists in the
'node/basic' path.  This 'basicnode' package is the equivalent of what
was formerly called 'ipldfree' on the master branch.

The demo codegen content is now under 'node/gendemo'.
Note that it is BROKEN in this commit; there was too much there to
clean up and address in one commit, and this one has already become
quite large and includes a bewildering number of moves as well as
finer content rearrangements.  Fixes will come in next commit(s).

(Why?  Well, the demo codegen content leaned on the basicnode types
for string and int.  Turns out this actually opens *quite* the
can of worms when you try to rectify it.
At the surface level, the basicnode package's types aren't exported.
Could we export them?  Well, sure.  Except that's only half the trick.
Can we export the *assemblers* for those types?  Well, sure.  But...
Can we export the 'w' fields for those assemblers for those types?
**NO** -- this is where it all breaks down -- if those fields are
exported, we lose all control over all immutability everywhere.
Shoot...?  Very shoot.
One clearly correct solution to this is to generate new types for
all of the primitive scalars in every codegen output package;
the downside, obviously, is that's a bit verbose in resulting code.
There might be another very alexandrian solution to this gordian knot:
exporting the basicnode package's types after all, but not their
assemblers; then leaning further into their implementation detail
of being implemented as typedefs rather than strictly enclosed structs,
and having every part of codegen types that handle primitive leafs
directly perform and manage the relevant casts.  The bigger picture
wisdom of this has not been completely analyzed yet.)

The 'impls' path has in general become 'node'.
This will stay this way when we finally merge over core on master;
all the current 'impl' (without the s; sigh) paths will become 'node'.

Numerous new HACKME files have appeared; some are splits of old ones
to account for the new package splits; some are hoisted and polished
from what were previously commentstorms in the code; some are
completely new.  There are also a few new package-scope godocs.

The benchmarks and tests are rearranged and much (much) improved.
Naming is more consistent; and things are consistently extracted
to the 'node/mixins/tests' package, and imported and used from there.
(If you want all the same benchmark info as before, you'll now have
to run benchmarks from several different packages.)