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/c-api/src/wasi.rs

@@ -254,7 +254,7 @@ fn create_wasi_ctx(config: wasi_config_t) -> Result<Rc<RefCell<WasiCtx>>> {
     for (dir, path) in config.preopens {
         builder = builder.preopened_dir(dir, path)?;
     }
-    Ok(Rc::new(RefCell::new(builder.build()?)))
+    Ok(Rc::new(RefCell::new(builder.build())))
 }
 
 #[repr(C)]