This PR adds support for erasing layout annotations when the --erase-jane-syntax flag is provided.

This includes parser standard changes:

• Parser standard is changed to parse layout annotation on type declarations as attribute names prefixed with jane.erasable.layouts to have them dropped during ast normalization. Implementation here differs from the jane syntax encoding used by the flambda-backend parser. This is meant as a temporary solution before we can do straightforward parser imports to make them the same again. The alternative of porting this PR doesn't seem to be worth the effort.
• Standard ast normalization is changed to allow float# --> float rewrites.

And parser extended changes:

• Parser extended is changed to drop all layout annotations during parsing (same as what we do for locals).
• Parser extended removes type alias nodes with no names completely (since (... as _) is invalid syntax)

This PR notably doesn't make any changes to [@@immediate] and [@@immediate64]. Those attributes remain the same through this process. See update

This PR should be merged after #60. Already merged

Update regarding [@@immediate] and [@@immediate64]

Last three commits added to this PR changes the treatment of these attributes.

Now ocamlformat rewrites both of these attributes into equivalent layout annotations by default. And when running with --erase-jane-syntax, the program does the opposite and rewrites immediate/immediate64 layout annotation on type declarations back into attributes.

Make sure this flambda-backend PR gets merged before this change to not break existing code. merged

Might be easier to review all the changes before this and then the immediate rewrite commits separately.