compiler: check arch-dependent implementation

Array lengths and indices are kept 32-bit no matter what arch is chosen;
pointer types and word sizes are in accordance with the arch chosen.
[skip bamboo]

compiler.dargent: fix simpl rules & remove unused code

compiler.dargent: WIP - typecheck for layout polymorphism

- have typecheck working for some basic cases
- change core lang to allow layout args
- introduce TLApp to surface syntax

[skip lemma]

compiler: add flags to specify Arch

[skip lemma]

compiler.dargent: fix code gen for array elements

[skip lemma]

dargent: rewrite allocations and darget type checking

Mostly moving allocations into an abstract type so we can keep
invariants about them

compiler: rearrange dargent things so they make more sense

[skip ci][skip lemma]