wasm-smith: add --module-shape option.

[mobsceneZ]

This PR fixes #2250 and the key changes are summarized as follows:

• Add a new option called module_shape into wasm-smith which automatically disables available_imports and exports when enabled;
• For simplicity, we handle the imports and exports declared in the shape module together in a single function: _imports_exports_from_module_shape. And this function presents two notable differences when compared with function _arbitrary_imports_from_available and _required_exports:
  1. To adhere more closely to the meaning of "module shape", it copies all declared imports into the target module, rather than randomly selecting a subset. As a result, defined limits such as max_funcs may be exceeded;
  2. Relying on wasmparser::Validator for type information, as the _required_exports does, would introduce the inconvenient step of converting CoreTypeId back to ModuleIndex. To avoid this, this function directly copies the entire type section from the shape module to the target module, which also aligns with the implementation of _arbitrary_imports_from_available;

Please feel free to share any suggestions/concerns.

[dicej]

Thanks for doing this! Looks good to me overall; please see the comment inline about panicking on error.