only wasi_cap_std_sync and wasi_tokio need to define WasiCtxBuilders (#2917)

* wasmtime-wasi: re-exporting this WasiCtxBuilder was shadowing the right one

wasi-common's WasiCtxBuilder is really only useful wasi_cap_std_sync and
wasi_tokio to implement their own Builder on top of.

This re-export of wasi-common's is 1. not useful and 2. shadow's the
re-export of the right one in sync::*.

* wasi-common: eliminate WasiCtxBuilder, make the builder methods on WasiCtx instead

* delete wasi-common::WasiCtxBuilder altogether

just put those methods directly on &mut WasiCtx.

As a bonus, the sync and tokio WasiCtxBuilder::build functions
are no longer fallible!

* bench fixes

* more test fixes

crates/wasi-common/src/lib.rs

@@ -64,7 +64,7 @@ pub mod table;
 
 pub use cap_rand::RngCore;
 pub use clocks::{SystemTimeSpec, WasiClocks, WasiMonotonicClock, WasiSystemClock};
-pub use ctx::{WasiCtx, WasiCtxBuilder};
+pub use ctx::WasiCtx;
 pub use dir::WasiDir;
 pub use error::{Context, Error, ErrorExt, ErrorKind};
 pub use file::WasiFile;