More tests & other housekeeping

[pufferfish101007]

wip.

• instructions_test! restyled & restructured - easier to read/write and allow project setup
• tests for more instructions (hopefully all, other than ones expected to be present only in special steps)
• test that Display implementations produce valid json (this has caused annoyance before)

and also

• bump rust nightly version

This gets test coverage up from ~40% to ~50% (according to cargo-llvm-cov) which isn't bad considering that most of src/wasm, src/ir and src/optimisation isn't tested at all.

Still to do: test const folding somehow.