fix C compilation errors with closures and nested types

[zerbina]

Summary

Fix a bug with hook synthesis that led to C compiler errors when using some closure procedures or refs of non-top-level object type.

Details

• sighashes now uses the symbol ID as the object representation for anonymous object/enum types and object/enum types not defined at the top-level
• this means that hashType(x) != hashType(y) when sameType(x, y) == false, for object and enum types
• as a consequence, unique hook procedures are used for ref T types that used the aforementioned types for T
• RTTI, which is assigned to types based on type hashes, also uses unique instances per object type now

Multiple same-shaped anonymous/non-top-level object types are not merged into a single one by the C code generator, which due to all sharing the same set of hook procedures, resulted in C compiler errors due to implicit conversions between incompatible pointer types.

[saem]

/merge

[github-actions[bot]]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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## Notes for Reviewers * a latent issue that was surfaced by #1382 * I've built the tests locally with `-Werror=incompatible-pointer-types` enabled to make sure they compile