Optimise our sum representation

[hugrbot]

Originally raised by @doug-q:

Currently we represent a sum as a struct with a u32 tag field and a field for each variant. Each variant field of the sum struct is a struct with a field per variant field. For example, a bool is {u32, {}, {}}, a int + float is {u32, {i64}, {f64}}. The tag field is elided for sums of a single variant. This is all implemented in sum.rs and the rest of the codebase SHOULD be agnostic as to the representation.

There are three avenues of optimisation:

• Elide empty structs, elide the struct wrapper of single-field-variants, elide the outer sum struct when all variants are empty
• Use the minimum-possible width for the tag field. in particular, for bool this is i1. and together with the previous optimisation, this makes the representation of bool i1.
• We store all of the possible variants, with the unused ones poison. Instead, store a byte array of the length of the longest variant