feat: memory64 support

[logan-gatlin]

Two small changes:

• Stop truncating MemArg offsets to 32 bits
• Allow ConstExpr::Extended when accessing data segments and tables. This is not strictly memory64 related, but the memory64 spec-tests require it, and its a trivial change.

This change is not backwards compatible, MemArg.offset has changed from a u32 to a u64.

[guybedford]

Added a commit to fix a pre-existing bug in src/passes/used.rs that becomes more relevant with this PR's acceptance of ConstExpr::Extended in data/element validation.

Problem: The GC used-set pass only matched ConstExpr::Global and ConstExpr::RefFunc when tracking references in:

• Data segment offsets (line ~238)
• Element segment offsets (line ~268)
• Element expression items (lines ~251-264)

If any of these used a ConstExpr::Extended containing GlobalGet or RefFunc operations, those references would not be marked as used, potentially causing the GC pass to incorrectly remove globals or functions that are still referenced.

Fix: Extracted a push_const_expr_refs helper that exhaustively handles all ConstExpr variants (including Extended) and applied it uniformly across globals, data segment offsets, element offsets, and element expression items. This also simplifies the existing globals handling as a nice side effect.