Implement RFC 11: Redesigning Wasmtime's APIs (#2897)

Implement Wasmtime's new API as designed by RFC 11. This is quite a large commit which has had lots of discussion externally, so for more information it's best to read the RFC thread and the PR thread.

examples/threads.wat

@@ -1,5 +1,4 @@
 (module
-    (func $message (import "" "hello") (param i32))
-    (global $id (import "" "id") i32)
-    (func (export "run") (call $message (global.get $id)))
+    (func $hello (import "global" "hello"))
+    (func (export "run") (call $hello))
 )