[WASM] Initial runtime library

Add directory libraries/stdlib/wasm as a placeholder for WASM runtime library
Copy built-ins from core/builtins
Add ExcludeFromCodegen annotation for internal needs

libraries/stdlib/wasm/builtins/README.md

@@ -0,0 +1,6 @@
+This directory is a modified copy of `core/builtins` adapted for current
+ needs of WASM backend.
+ 
+This is a temporary solution for a development convenience. Most of files 
+from `core/builtins` will be reused once compiler implementation and
+stdlib become stable.