Major API-incompatible changes include:
- Introduce TrapCode::UnreachableCodeReached, used for unreachable in wasm.
- cranelift-wasm's `declare_signature` now takes its signature by value
- cranelift-wasm's `declare_table_elements` `elems` parameter now takes a boxed slice
- Remove cranelift-wasm's `ModuleEnvironment`'s `get_signature`,
`get_num_func_imports`, `get_func_type`, and `get_global`.
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