witx: add support for parsing multiple top-level files

[pchickey]

This PR adds support in witx to parse multiple top-level witx files. This is an api-breaking change - public functions that previously took an impl AsRef<Path> to a witx file now take an &[impl AsRef<Path>]

There is still only one global namespace for all type names and module names across all files. This means that if two top-level files define the same name, the second definition will cause an error, just like it would if both definitions are in the same file. However, if two top-level files depend each depend on a third file which defines a name, the name will only be defined once, and each reference to that name will be equal.

I'd like to fix the scoping issue at some point soon, but for now this unblocks #98.

[sunfishcode]

I agree, this looks like a good approach for now. The one thing that would be good to add here is a test for the error case -- when a given name is defined as one type in one file, and another type in another, so that even though we have a single global namespace for these types, we'll catch errors if we have collisions.

[pchickey]

Great idea - done :)

[sunfishcode]

Looks good, thanks!