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/include/wasi.h

@@ -52,6 +52,7 @@ WASI_DECLARE_OWN(instance)
 
 WASI_API_EXTERN own wasi_instance_t* wasi_instance_new(
   wasm_store_t* store,
+  const char* name,
   own wasi_config_t* config,
   own wasm_trap_t** trap
 );