Pre-initialised UTF string constants with `memory.data<>`

[geraintluff]

Feature suggestion

I'd love a syntax like this:

let utf8Ptr = memory.data<u8>("Hello");
let utf16Ptr = memory.data<u16>("Hello");

At the moment I'm doing this at runtime instead:

let bytes = String.UTF8.byteLength(str, true);
let utf8Ptr = heap.alloc(bytes);
String.UTF8.encodeUnsafe(changetype<usize>(str), str.length, utf8Ptr, true);

I could use String.UTF8.encode() instead, and then store the ArrayBuffer so it doesn't get GC'd, but as far as I can tell, that still ends up embedding the UTF-16 string in the output.