Implement embedded FS directly with strings

[jakebailey]

In profiling #275, I discovered that a good chunk of time and memory allocation is due to the embedded FS. Our VFS interface reads file contents in terms of strings, but the underlying io/fs.FS interface uses mutable byte slices, so that incurs a copy for each file read. If you keep going down, it turns out that go:embed is implemented in terms of immutable in-binary strings, which are then copied out of byte slices! So, each time we access the embedded VFS, we are converting a string to a byte slice, just to turn it back into a string.

If we skip the io/fs.FS layer and instead directly implement our own vfs.FS interface in terms of go:embed'd strings, we can skip this entirely.

On my machine, this brings the total runtime of internal/testutil/runner from 9.3s to 6.3s, reducing memory allocation during the test from 42.7GB to 10.25GB.

I'm not 100% happy with the implementation itself (mainly if we choose to add more stuff, though I doubt I'd do the translations as a FS layer), but, I think it's okay for now.