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/wasi-common/src/pipe.rs

@@ -32,9 +32,8 @@ use std::sync::{Arc, RwLock};
 /// let clocks = todo!();
 /// let sched = todo!();
 /// let table = Rc::new(RefCell::new(Table::new()));
-/// let ctx = WasiCtx::builder(random, clocks, sched, table)
-///             .stdin(Box::new(stdin.clone()))
-///             .build();
+/// let mut ctx = WasiCtx::new(random, clocks, sched, table);
+/// ctx.set_stdin(Box::new(stdin.clone()));
 /// ```
 #[derive(Debug)]
 pub struct ReadPipe<R: Read> {
@@ -203,9 +202,8 @@ impl<R: Read + Any + Send + Sync> WasiFile for ReadPipe<R> {
 /// let clocks = todo!();
 /// let sched = todo!();
 /// let table = Rc::new(RefCell::new(Table::new()));
-/// let ctx = WasiCtx::builder(random, clocks, sched, table)
-///             .stdout(Box::new(stdout.clone()))
-///             .build();
+/// let mut ctx = WasiCtx::new(random, clocks, sched, table);
+/// ctx.set_stdout(Box::new(stdout.clone()));
 /// // use ctx in an instance, then make sure it is dropped:
 /// drop(ctx);
 /// let contents: Vec<u8> = stdout.try_into_inner().expect("sole remaining reference to WritePipe").into_inner();