Simplify metadata/file-type handling in filesystem::primitives

[alexcrichton]

Avoid unwrapping structures into a shape only to re-wrap them later on into a WASI-defined shape. Instead remove various extension traits and shuffling of fields and instead use an enum for the definitions of Metadata and FileType which are either standard library types or rustix-based types. Accessors then delegate to the appropriate fields depending on how the data lies at rest.

The goal here is to remove layer of cognitive indirection where there's a platform-specific source of truth for the underlying data. Accessors where a specific type is requested or a particular operation is where the interpretation of the underlying data happens then.

Note that the reason that std::fs::Metadata alone isn't sufficient is that the Rust standard library doesn't bind functions like statat and there's no way to create a std::fs::Metadata externally from the standard library, hence the Unix-specific layer where metadata might be std::fs::Metadata and might be rustix::fs::Stat.