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/traps.rs

@@ -500,7 +500,7 @@ fn parse_dwarf_info() -> Result<()> {
         &store,
         wasmtime_wasi::sync::WasiCtxBuilder::new()
             .inherit_stdio()
-            .build()?,
+            .build(),
     )
     .add_to_linker(&mut linker)?;
     linker.module("", &module)?;