Use NotImplemented as the infer-default sentinel; free None as a literal value

[behdad]

Problem

f0495bc ("Support inferred defaults from boundary values") repurposed
default=None to mean "infer the default". That collided with None being a
legitimate value: HarfBuzz's decomposition (dm) dataset packs a dict
with default=None and a mapping that sends None -> 0 ("no decomposition").
After that commit, regenerating HarfBuzz's tables fails:

ValueError: default=None is only supported for list input

This was a latent break since f0495bc; it surfaces whenever HarfBuzz is
regenerated against current packtab.

Fix

Move the inference request off None and onto the explicit sentinel
NotImplemented, restoring None as an ordinary value:

pack_table(list_data, default=NotImplemented)    # infer from boundary values
pack_table(dict_data, default=None, mapping=m)    # None is a real, mapped value

• None is now mapped like any other value (e.g. None -> 0).
• Inference is requested with default=NotImplemented (list input only; dict + NotImplemented still raises).
• Default remains 0, so existing callers are unaffected.
• In the Rust port this is naturally expressed with Option; NotImplemented
  is the Python equivalent now that None is a real value.

Also included

• Repair Code.print_h, which referenced a nonexistent Function.linkage and
  would raise AttributeError if called. It now emits valid C prototypes for
  the public functions. Not on any generated-code path — no output changes.

Verification

• pytest: 231 passing (updated inference tests to NotImplemented; added
  tests that None is a literal value with a mapping, and that dict + NotImplemented raises).
• End-to-end: make -f update-unicode-tables.make clean all in HarfBuzz now
  succeeds, and hb-ucd-table.hh regenerates byte-identical to the
  committed table.

🤖 Generated with Claude Code