Use the stdlib Hashtbl for the compiler's hash sets

[cknitt]

Second of four PRs moving the compiler's own collections onto the OCaml standard library, stacked on #8786. This one covers hash sets.

Hash_set_ident, Hash_set_string and Used_attributes.Attribute_name_set are now unit tables from Hashtbl.Make, using the same equality and hash functions as before. Lam_module_ident.Hash_set is an alias of Lam_module_ident.Hash. Hash_set and Hash_set_gen are removed, together with their ounit tests.

• Keep-first adds. Every add is guarded by mem, so the first key added is kept. This matters for Lam_module_ident: two module ids can be equal while carrying different ids, and the first one decides the name the import is bound to. Lam_module_ident.set_add does this for the module sets.
• Deterministic import order. The hard dependencies were sorted by module name only, so ties fell back to hash-set order. Ties happen when the same module is imported both with and without default. They are now broken by kind and then default. The one change in the generated output is key_word_property.mjs. There the old order put default first for one module and last for another; now the plain import always comes first.

Output: byte-identical to #8786 on 627 of 628 files; the exception is key_word_property.res, described above.

Performance: neutral (CPU +0.2%, allocation −0.1% vs master). Earlier figures that suggested a speedup didn't reproduce.

🤖 Generated with Claude Code

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commit: 9b8ce57

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.64%. Comparing base (ccb7c23) to head (9b8ce57).

Files with missing lines	Patch %	Lines
compiler/core/lam_module_ident.ml	75.00%	2 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##           stdlib-hashtbl    #8787      +/-   ##
==================================================
- Coverage           80.66%   80.64%   -0.03%     
==================================================
  Files                 461      458       -3     
  Lines               62630    62523     -107     
==================================================
- Hits                50520    50420     -100     
+ Misses              12110    12103       -7     

Files with missing lines	Coverage Δ
compiler/core/js_fold_basic.ml	100.00% <ø> (ø)
compiler/core/lam_check.ml	95.00% <100.00%> (ø)
compiler/core/lam_coercion.ml	97.50% <100.00%> (ø)
compiler/core/lam_compile_env.ml	84.44% <100.00%> (ø)
compiler/core/lam_compile_main.ml	89.87% <100.00%> (+0.16%)	⬆️
compiler/core/lam_dce.ml	92.10% <ø> (ø)
compiler/ext/hash_set_ident.ml	100.00% <ø> (ø)
compiler/ml/used_attributes.ml	100.00% <100.00%> (ø)
tests/ounit_tests/ounit_tests_main.ml	100.00% <ø> (ø)
compiler/core/lam_module_ident.ml	90.90% <75.00%> (-9.10%)	⬇️

... and 1 file with indirect coverage changes

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