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/wig/src/utils.rs

@@ -1,4 +1,5 @@
 use proc_macro2::{Literal, TokenStream, TokenTree};
+
 /// Given the input tokens to a macro invocation, return the path to the
 /// witx file to process.
 pub(crate) fn witx_path_from_args(args: TokenStream) -> (String, String) {
@@ -26,8 +27,8 @@ pub(crate) fn witx_path_from_args(args: TokenStream) -> (String, String) {
 }
 
 fn witx_path(phase: &str, id: &str) -> String {
-    let root = std::env::var("CARGO_MANIFEST_DIR").unwrap_or(".".into());
-    format!("{}/WASI/phases/{}/witx/{}.witx", root, phase, id)
+    let root = env!("CARGO_MANIFEST_DIR");
+    format!("{}/../WASI/phases/{}/witx/{}.witx", root, phase, id)
 }
 
 // Convert a `Literal` holding a string literal into the `String`.