Enable the new preview1 implementation by default

[alexcrichton]

Currently Wasmtime has two implementations of the wasi_snapshot_preview1 set of APIs. The now-historical implementation lives in the wasi-common crate and the more recent implementation lives in the wasmtime-wasi crate. The main difference is that the wasmtime-wasi implementation is based on the implementation of preview2, meaning that the preview1 implementation is a shim in that direction. Additionally currently the preview2 implementation of preview1 is accessible via the -Spreview2 flag on the CLI.

This commit updates the interpretation of the -Spreview2 flag and the defaults around which implementation to choose. By default the preview1-built-on-preview2 implementation (the new wasmtime-wasi implementation) is selected now. This means that the wasi-common implementation is disabled by default. There are still two use cases to keep running the wasi-common implementation, however:

• When running modules that import from wasi_unstable, the "snapshot" before wasi_snapshot_preview1, currently wasi-common is required. The shims to implement wasi_unstable have not yet been implemented in the wasmtime-wasi crate.

• When running with WASI threads (-Sthreads) the preview2 implementation does not work. This is because the preview2 implementation expects mutable access to the table which is not granted when threads are enabled.

Tests using wasi_unstable now pass -Spreview2=n to explicitly request the old wasi-common implementation. Additionally the wasi-common implementation is still selected by default when -Sthreads is passed (enabling the WASI threads proposal).

[alexcrichton]

I'll also note that I've included a few minor fixes to wasmtime-wasi's implementation of preview1 to fix some failing tests when it was enabled.

[pchickey]

Thanks. This is an exciting move! If we survive a release of this, I'll start moving the old implementation to be exported by an off-by-default wasmtime_wasi::legacy mod, and move ::preview2 to the root.