Missing `xShm`* bindings

[bkoropoff]

These would be nice to allow WAL support. Exposing a safe interface to Rust would require some thought, since SQlite does not explicitly inform you when it no longer references the returned memory region. And, of course, mapping memory in Rust is inherently unsafe on many platforms because accessing the mapped region will fault if the underlying file has been truncated by an external process, which is fun.

[carlsverre]

This is a good idea, but as you point out, it's tricky to implement. If absolutely needed, we can either expose them as unsafe methods on the vfs, or we can figure out how to pass through to the parent vfs as I do for dlopen (although slightly more tricky due to them being on io_methods).