[WIP] Refactoring RefLayout

[btwj]

The RefLayout{Get/Set}Field opcodes (and their repeated counterparts) would read/write one byte at a time.
This PR introduces two new Opcodes that are created during normalization:

• ByteArrayGetField[offset: int]<T>(arr: Array<byte>, i_offset: int) -> T
• ByteArraySetField[offset: int]<T>(arr: Array<byte>, i_offset: int, value: T) -> void

All RefLayout reads/writes (including those for repeated fields) are normalized into this form.

When lowering to machine code, we decompose these loads/stores into power-of-two loads/stores (e.g. a u56, with 7 bytes, will be decomposed into loads of 4+2+1 bytes).

Todo:

• Since there is no intermediate lowering for JVM code generation, add a normalizer flag to perform byte-by-byte loads/stores (same as the old behavior) specially for the JVM target.

[titzer]

As discussed, I think you only need to worry about handling partial reads for the last tn.sub[i]. You can over-read for a field that is not too close to the end of a struct. We can do that optimization later and control it with a Fact set on the instruction.

[titzer]

lgtm other than small comments