Reduce boilerplate in wasmtime-wasi (#707)

This commit uses the `*.witx` files describing the current wasi API to
reduce the boilerplate used to define implementations in the
`wasmtime-wasi` crate. Eventually I'd like to remove lots of boilerplate
in the `wasi-common` crate too, but this should at least be a good start!

The boilerplate removed here is:

* No need to list each function to add it to the
  `wasmtime_runtime::Module` being created

* No need to list the signature of the function in a separate
  `syscalls.rs` file.

Instead the `*.witx` file is processed in a single-use macro inside the
`wasmtime-wasi` crate. This macro uses the signatures known from
`*.witx` to automatically register with the right type in the wasm
module as well as define a wrapper that the wasm module will call into.
Functionally this is all the same as before, it's just defined in a
different way now!

The shim generated by this macro which wasmtime calls into only uses
`i32`/`i64`/etc wasm types, and it internally uses `as` casts to convert
to the right wasi types when delegating into the `wasi-common` crate.

One change was necessary to get this implemented, however. The functions
in `wasi-common` sometimes took `WasiCtx` and sometimes took a slice of
memory. After this PR they uniformly all require both `WasiCtx` and
memory so the wrappers can be auto-generated. The arguments are ignored
if they weren't previously required.

crates/wasi-common/src/hostcalls_impl/misc.rs

@@ -128,6 +128,7 @@ pub(crate) fn environ_sizes_get(
 }
 
 pub(crate) fn random_get(
+    _wasi_ctx: &WasiCtx,
     memory: &mut [u8],
     buf_ptr: wasi32::uintptr_t,
     buf_len: wasi32::size_t,
@@ -143,6 +144,7 @@ pub(crate) fn random_get(
 }
 
 pub(crate) fn clock_res_get(
+    _wasi_ctx: &WasiCtx,
     memory: &mut [u8],
     clock_id: wasi::__wasi_clockid_t,
     resolution_ptr: wasi32::uintptr_t,
@@ -161,6 +163,7 @@ pub(crate) fn clock_res_get(
 }
 
 pub(crate) fn clock_time_get(
+    _wasi_ctx: &WasiCtx,
     memory: &mut [u8],
     clock_id: wasi::__wasi_clockid_t,
     precision: wasi::__wasi_timestamp_t,
@@ -180,7 +183,7 @@ pub(crate) fn clock_time_get(
     enc_timestamp_byref(memory, time_ptr, time)
 }
 
-pub(crate) fn sched_yield() -> Result<()> {
+pub(crate) fn sched_yield(_wasi_ctx: &WasiCtx, _memory: &mut [u8]) -> Result<()> {
     trace!("sched_yield()");
 
     std::thread::yield_now();