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/misc/rust/macro/src/lib.rs

@@ -61,7 +61,7 @@ fn generate_load(item: &syn::ItemTrait) -> syn::Result<TokenStream> {
 
             let mut imports: Vec<Extern> = Vec::new();
             if let Some(module_name) = data.find_wasi_module_name() {
-                let wasi_cx = #root::wasmtime_wasi::WasiCtxBuilder::new().build()?;
+                let wasi_cx = #root::wasmtime_wasi::WasiCtxBuilder::new().build();
                 let wasi = #root::wasmtime_wasi::Wasi::new(&store, wasi_cx);
                 for i in module.imports().iter() {
                     if i.module() != module_name {