Add Memory64 support (WASM 3.0)

[avrabe]

Summary

WRT references WASM 3.0 but doesn't support Memory64 yet. The testsuite has memory64 tests that will fail.

Current State

• Type system: Limits.memory64 field exists but unused at runtime
• Runtime: All memory addresses use u32 (casts like Value::I32(addr) => *addr as u32)
• Tests: memory64.wast, load64.wast, etc. exist in testsuite

Implementation Approach

Use u64 for all memory addresses:

• Simple, consistent implementation
• Modern 64-bit CPUs handle u64 as efficiently as u32
• No branching overhead from address type checking
• Truncate to u32 only at boundary when writing to 32-bit memories

Tasks

1. Update MemoryLoad/MemoryStore to use u64 addresses
2. Add Value::I64 address handling in memory operations
3. Update memory.size/memory.grow for 64-bit memories
4. Update bulk memory operations (copy, fill, init)
5. Enable memory64 WAST tests