Improve inlining where locals can be aliased; Add a crude mechanism for explicit unchecked array accesses

The `unchecked` builtin should be handled with ultimate care and it would be a lot better if there'd be a mechanism doing this automatically.

std/assembly/builtins.ts

@@ -83,6 +83,8 @@ export declare function abort(
   columnNumber?: u32
 ): void;
 
+export declare function unchecked<T>(expr: T): T;
+
 export declare function i8(value: void): i8;
 export namespace i8 {
   export const MIN_VALUE: i8 = -128;