Remove the device and inode numbers from the API.

As discussed [here], remove the fields which correspond to `st_dev`, `st_ino`,
and `d_ino` in POSIX from the stat and directory entry structs.

 - Device numbers assume the existence of a global device number space,
   which creates implicit relationships between otherwise unrelated
   components.

 - Not all filesystem implementations have these numbers. And some that
   do have these numbers require extra implementation cost to retrieve them.

 - These numbers leak potentially sensitive or identifying information from the
   underlying filesystem implementation.

In their place, provide some functions, `is-same-object`,
`metadata-hash`, and `metadata-hash-at`, for explicitly testing whether two
handles are the same file or have the same metadata, respectively. This doesn't
cover all possible use cases for device and inode numbers, but we can
add more functions as need arises.

[here]: WebAssembly#65 (comment)