Update WebAssembly C API submodule to latest commit. (#2579)

* Update WebAssembly C API submodule to latest commit.

This commit updates the WebAssembly C API submodule (for `wasm.h`) to the
latest commit out of master.

This fixes the behavior of `wasm_name_new_from_string` such that it no longer
copies the null character into the name, which caused unexpected failures when
using the Wasmtime linker as imports wouldn't resolve when the null was
present.

Along with this change were breaking changes to `wasm_func_call`, the host
callback signatures, and `wasm_instance_new` to take a vector type instead of a
pointer to an unsized array.

As a result, Wasmtime language bindings based on the C API will need to be
updated once this change is pulled in.

Fixes #2211.
Fixes #2131.

* Update Doxygen comments for wasm.h changes.

crates/c-api/src/vec.rs

@@ -4,6 +4,7 @@ use crate::{
     wasm_moduletype_t, wasm_tabletype_t, wasm_val_t, wasm_valtype_t,
 };
 use std::mem;
+use std::mem::MaybeUninit;
 use std::ptr;
 use std::slice;
 
@@ -54,6 +55,18 @@ macro_rules! declare_vecs {
                 }
             }
 
+            pub fn as_uninit_slice(&mut self) -> &mut [MaybeUninit<$elem_ty>] {
+                // Note that we're careful to not create a slice with a null
+                // pointer as the data pointer, since that isn't defined
+                // behavior in Rust.
+                if self.size == 0 {
+                    &mut []
+                } else {
+                    assert!(!self.data.is_null());
+                    unsafe { slice::from_raw_parts_mut(self.data as _, self.size) }
+                }
+            }
+
             pub fn take(&mut self) -> Vec<$elem_ty> {
                 if self.data.is_null() {
                     return Vec::new();