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

tests/all/host_funcs.rs

@@ -757,7 +757,7 @@ fn wasi_imports() -> Result<()> {
     let engine = Engine::new(&config)?;
     let module = Module::new(&engine, wasm)?;
     let store = Store::new(&engine);
-    assert!(Wasi::set_context(&store, WasiCtxBuilder::new().build()?).is_ok());
+    assert!(Wasi::set_context(&store, WasiCtxBuilder::new().build()).is_ok());
     let linker = Linker::new(&store);
     let instance = linker.instantiate(&module)?;