Add `From` implementations for structural types to `StructuralType`, `SubType` and `Type`

Looking at all these conversions between `Type`, `SubType`, `StructuralType`, and each structural type variant, it seems like we could have some more `From` implementations to make this less noisy and more automatic.

To really benefit from this, and because `From` is not transitive by default, I think we would want all of these impls:

```rust
impl From for StructuralType { ... }
impl From for StructuralType { ... }
impl From for StructuralType { ... }

impl From for SubType { ... }
impl From for SubType { ... }
impl From for SubType { ... }
impl From for SubType { ... }

impl From for Type { ... }
impl From for Type { ... }
impl From for Type { ... }
impl From for Type { ... }
impl From for Type { ... }
```

_Originally posted by @fitzgen in https://github.com/bytecodealliance/wasm-tools/pull/1059#discussion_r1230108885_