wasm: Implement 'field_ptr' constants

This implements the `field_ptr` value for pointers. As the value only provides us with the index,
we must calculate the offset from the container type using said index. (i.e. the offset from a struct field at index 2).

Besides this, small miscellaneous fixes/updates were done to get remaining behavior tests passing:
 - We start the function table index at 1, so unresolved function pointers don't can be null-checked properly.
 - Implement genTypedValue for floats up to f64.
 - Fix zero-sized arguments by only creating `args` for non-zero-sized types.
 - lowerConstant now works for all decl_ref's.
 - lowerConstant properly lowers optional pointers, so `null` pointers are lowered to `0`.

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