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

benches/instantiation.rs

@@ -11,7 +11,7 @@ fn instantiate(module: &Module) -> Result<Instance> {
     // As we don't actually invoke Wasm code in this benchmark, we still add
     // the WASI context to the store as it is considered part of getting a
     // module that depends on WASI "ready to run".
-    Wasi::set_context(&store, WasiCtxBuilder::new().build()?)
+    Wasi::set_context(&store, WasiCtxBuilder::new().build())
         .map_err(|_| anyhow::anyhow!("wasi set_context failed"))?;
 
     let linker = Linker::new(&store);