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/fs.rs

@@ -14,7 +14,11 @@ use std::fs::File;
 use std::io::{self, Read, Seek, SeekFrom, Write};
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
-pub(crate) unsafe fn fd_close(wasi_ctx: &mut WasiCtx, fd: wasi::__wasi_fd_t) -> Result<()> {
+pub(crate) unsafe fn fd_close(
+    wasi_ctx: &mut WasiCtx,
+    _memory: &mut [u8],
+    fd: wasi::__wasi_fd_t,
+) -> Result<()> {
     trace!("fd_close(fd={:?})", fd);
 
     if let Ok(fe) = wasi_ctx.get_fd_entry(fd) {
@@ -28,7 +32,11 @@ pub(crate) unsafe fn fd_close(wasi_ctx: &mut WasiCtx, fd: wasi::__wasi_fd_t) ->
     Ok(())
 }
 
-pub(crate) unsafe fn fd_datasync(wasi_ctx: &WasiCtx, fd: wasi::__wasi_fd_t) -> Result<()> {
+pub(crate) unsafe fn fd_datasync(
+    wasi_ctx: &WasiCtx,
+    _memory: &mut [u8],
+    fd: wasi::__wasi_fd_t,
+) -> Result<()> {
     trace!("fd_datasync(fd={:?})", fd);
 
     let fd = wasi_ctx
@@ -173,6 +181,7 @@ pub(crate) unsafe fn fd_read(
 
 pub(crate) unsafe fn fd_renumber(
     wasi_ctx: &mut WasiCtx,
+    _memory: &mut [u8],
     from: wasi::__wasi_fd_t,
     to: wasi::__wasi_fd_t,
 ) -> Result<()> {
@@ -289,6 +298,7 @@ pub(crate) unsafe fn fd_fdstat_get(
 
 pub(crate) unsafe fn fd_fdstat_set_flags(
     wasi_ctx: &WasiCtx,
+    _memory: &mut [u8],
     fd: wasi::__wasi_fd_t,
     fdflags: wasi::__wasi_fdflags_t,
 ) -> Result<()> {
@@ -304,6 +314,7 @@ pub(crate) unsafe fn fd_fdstat_set_flags(
 
 pub(crate) unsafe fn fd_fdstat_set_rights(
     wasi_ctx: &mut WasiCtx,
+    _memory: &mut [u8],
     fd: wasi::__wasi_fd_t,
     fs_rights_base: wasi::__wasi_rights_t,
     fs_rights_inheriting: wasi::__wasi_rights_t,
@@ -327,7 +338,11 @@ pub(crate) unsafe fn fd_fdstat_set_rights(
     Ok(())
 }
 
-pub(crate) unsafe fn fd_sync(wasi_ctx: &WasiCtx, fd: wasi::__wasi_fd_t) -> Result<()> {
+pub(crate) unsafe fn fd_sync(
+    wasi_ctx: &WasiCtx,
+    _memory: &mut [u8],
+    fd: wasi::__wasi_fd_t,
+) -> Result<()> {
     trace!("fd_sync(fd={:?})", fd);
 
     let fd = wasi_ctx
@@ -381,6 +396,7 @@ pub(crate) unsafe fn fd_write(
 
 pub(crate) unsafe fn fd_advise(
     wasi_ctx: &WasiCtx,
+    _memory: &mut [u8],
     fd: wasi::__wasi_fd_t,
     offset: wasi::__wasi_filesize_t,
     len: wasi::__wasi_filesize_t,
@@ -404,6 +420,7 @@ pub(crate) unsafe fn fd_advise(
 
 pub(crate) unsafe fn fd_allocate(
     wasi_ctx: &WasiCtx,
+    _memory: &mut [u8],
     fd: wasi::__wasi_fd_t,
     offset: wasi::__wasi_filesize_t,
     len: wasi::__wasi_filesize_t,
@@ -687,6 +704,7 @@ pub(crate) unsafe fn fd_filestat_get(
 
 pub(crate) unsafe fn fd_filestat_set_times(
     wasi_ctx: &WasiCtx,
+    _memory: &mut [u8],
     fd: wasi::__wasi_fd_t,
     st_atim: wasi::__wasi_timestamp_t,
     st_mtim: wasi::__wasi_timestamp_t,
@@ -746,6 +764,7 @@ pub(crate) fn fd_filestat_set_times_impl(
 
 pub(crate) unsafe fn fd_filestat_set_size(
     wasi_ctx: &WasiCtx,
+    _memory: &mut [u8],
     fd: wasi::__wasi_fd_t,
     st_size: wasi::__wasi_filesize_t,
 ) -> Result<()> {