Add WasiCtxBuilder::preopened_handle to allow more flexible VFS usage

[joshuawarner32]

This will allow third-party libraries to implement their own VFS to expose to a wasi binary, such as (for example) VFS implemenations that record all reads/writes done from wasi, or purely user-space implementations of traditional filesystems.

Also modify the wasi-fs example to use this simplified & more powerful system.

(partial fix for #2232)

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[joshuawarner32]

@sunfishcode I'm happy to apply the refactoring you suggested above, but I'm looking for some input on which direction to take things.

[cardoso]

Considering it's been 2+ years, is this possible already without this PR?

[pchickey]

Yes this has been possible in wasi-common for a few years now.

Note that this functionality is going away in the preview 2 implementation, in favor of using component virtualization - a VFS can be implemented in WebAssembly now, instead of native code.

[alexcrichton]

I'm going to close this as this is quite an old PR at this point and I believe the previous summary is still applicable.