wasi-filesystem: delete access modes

[pchickey]

Implements the change for WebAssembly/wasi-filesystem#139: Remove the definition of access modes from the wasi-filesystem interface, the three functions for reading and modifying those modes, and the modes argument from open_at.

We never made time to implement these in Wasmtime, so the implementations deleted are all just todo!()s. We will add these functions back to wasi-filesystem in a future spec revision and accompany them with an implementation.

[alexcrichton]

Question on this, if we know we want to add this in the future should we be trying to future-proof this function? For example we could add an options: option<open-options> with a currently empty resource open-options or something like that.

Alternatively we could add a feature to the component model and WIT for something like keyword arguments with defaults. That's a much longer- term vision though.

[pchickey]

I believe that we can eliminate the modes parameter here and let open-at continue to do the reasonable default behavior, and then when users of the re-introduced modes stuff want to opt into a non-default behavior for modes, we can expose a new open-at-modes function that has the additional argument. Then when it comes time to make a breaking change in preview 3 we can consolidate down to just one open-at with mode arguments.

We could get fancier than that, but I expect the above will work fine without introducing much complexity.