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/wasmtime/src/lib.rs

@@ -202,7 +202,7 @@
 //! // Create an instance of `Wasi` which contains a `WasiCtx`. Note that
 //! // `WasiCtx` provides a number of ways to configure what the target program
 //! // will have access to.
-//! let wasi = Wasi::new(&store, WasiCtxBuilder::new().inherit_stdio().build()?);
+//! let wasi = Wasi::new(&store, WasiCtxBuilder::new().inherit_stdio().build());
 //! wasi.add_to_linker(&mut linker)?;
 //!
 //! // Instantiate our module with the imports we've created, and run it.