experiment: support migration via extended stable interfaces

[crusso]

builds on #4829 and #4812, addressing syntactic stable signatures.

A stable signature is now either a single interface of stable fields (as before) or (the new bit) a dual interface recording the pre and post upgrade interface when performing migration.

The pre and post fields are implicitly identical for a singleton interface and provide for backwards compatibility and ordinary upgrades (sans explicit migration).

Stability compatibility now checks the post interface of old module is compatible with the pre-interface of the new module.

<stab_sig> ::=
  <typ_dec>;* actor { <stab_field>;* }
  <typ_dec>;* actor ( { <stab_field>;* },  <stab_field>;* } )

Done this way, there should be no need to modify dfx, which defers to moc for the compatibility check anyway.

• fig grammar.sed to delete new production
• add some tests to test/cmp

[github-actions]

Comparing from 2b860e0 to f65d3f0:
In terms of gas, no changes are observed in 5 tests.
In terms of size, no changes are observed in 5 tests.