Add expression metadata

[MattWindsor91]

After one nightmarish attempt to hand-reduce a fuzzer expression too many, I'm once again considering adding metadata to expressions. This'll take quite a bit of heavy lifting even for the basic approach of changing t to 'm t.

My current thought process is that, instead of putting metadata on every sub-expression, we'd instead have a carrier expression (similar to an implicit block) that would coat the wrapped expression in some metadata. (Maybe, eventually, we'd backform the statement system so that it does the same?)

• Make type polymorphic
• Add metadata carrier leg to expression
• Devise some expression metadata tags