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/test-programs/tests/wasm_tests/runtime/cap_std_sync.rs

@@ -53,7 +53,7 @@ fn run(
         // cap-std-sync does not yet support the sync family of fdflags
         builder = builder.env("NO_FDFLAGS_SYNC_SUPPORT", "1")?;
 
-        let wasi = Wasi::new(&store, builder.build()?);
+        let wasi = Wasi::new(&store, builder.build());
 
         let mut linker = Linker::new(&store);
 

crates/test-programs/tests/wasm_tests/runtime/tokio.rs

@@ -62,7 +62,7 @@ fn run(
             // does not.
             builder = builder.env("NO_FDFLAGS_SYNC_SUPPORT", "1")?;
 
-            Wasi::set_context(&store, builder.build()?)
+            Wasi::set_context(&store, builder.build())
                 .map_err(|_| anyhow::anyhow!("wasi set_context failed"))?;
 
             let module =