Support WASI snapshot0 in the C API.

This commit adds support for snapshot0 in the WASI C API.

A name parameter was added to `wasi_instance_new` to accept which WASI module
is being instantiated.

Additionally, the C# API now supports constructing a WASI instance based on the
WASI module name.

Fixes #1221.

crates/c-api/src/ext.rs

@@ -185,7 +185,7 @@ pub unsafe extern "C" fn wasmtime_linker_define_wasi(
     instance: *const wasi_instance_t,
 ) -> bool {
     let linker = &mut (*linker).linker;
-    (*instance).wasi.add_to_linker(linker).is_ok()
+    (*instance).add_to_linker(linker).is_ok()
 }
 
 #[no_mangle]