Use the stdlib Map for the compiler's maps

[cknitt]

Last of four PRs moving the compiler's own collections onto the OCaml standard library, stacked on #8789. This one covers maps.

Map_ident, Map_int and Map_string are now Map.Make instances with the same comparison functions, including Ext_string.compare (length first) for strings, so iteration order is unchanged. Call sites use the stdlib API and argument order. The ext-only helpers map onto stdlib functions:

• find_exn → find
• find_default → find_opt with a default
• adjust → update
• disjoint_merge_exn → union with a failing merge
• to_sorted_array_with_f → bindings
• compare_key → Ext_string.compare

Ext_map and Map_gen are removed, together with the ounit tests of their helpers. With this PR the compiler has no hand-written ordered or hashed collections left apart from Ordered_hash_map and Hash_set_ident_mask, which have no stdlib equivalent.

Output: byte-identical to #8789 on 628 files (Belt, all of tests/tests/src, and the benchmark stress inputs).

Performance: the whole stack against master measures CPU +0.2% (noise) and allocation +0.6%. The removed maps had a compact leaf node that stdlib Map doesn't have.

🤖 Generated with Claude Code

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commit: 4d1e83e