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

examples/tokio/main.rs

@@ -146,7 +146,7 @@ async fn _run_wasm(inputs: Inputs) -> Result<(), Error> {
             .inherit_stdout()
             // Set an environment variable so the wasm knows its name.
             .env("NAME", &inputs.name)?
-            .build()?,
+            .build(),
     )
     .map_err(|_| anyhow!("setting wasi context"))?;