Store an array's element type in `ErasedType`

[Derppening]

A note for later: erased types should eventually store the erased type of an array's elements, as that stuff is not actually erased, and can be compiled to more efficient array reprs in both JS and WASM
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Typed_arrays

so we will probably need a specific ArrayType(eltType: ErasedValueType) form

Also see #504 (comment).