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/arraybuffer.ts

@@ -28,12 +28,3 @@ export class ArrayBuffer {
     return buffer;
   }
 }
-
-export declare interface FastArray<T> {}
-
-export declare interface ArrayBufferView<T> {
-  readonly buffer: ArrayBuffer;
-  readonly byteOffset: i32;
-  readonly byteLength: i32;
-  readonly length: i32;
-}