feat: Human-friendly metavariable printing

[croyzor]

Closes #3.

There are a couple of open questions here:

1. What's a better name than nameMap? I kind of like mnemonics as we use for the user-provided names of holes
2. What's your favourite factoring of the repeated req AskNames >>= \nm -> showRow nm pattern?

I have a hunch this could look nicer after we tackle #23.

New test case vec_length2.brat reveals an error, which I plan to fix in #38.
(This bit of code is driving me up the wall 😅)

[acl-cqc]

Naming. I reckon the current UserName should be QualName. Might as well be a newtype, rather than data, too, with the constructor also called QualName.

Then, the names here that we show to the user can be UserName...?

Failing that, hmmm, VarName? It's not good as many of our names can stand for variables. UserVarName is too long and directs me back down the QualName route. SourceCodeName, also too long - and we can't abbreviate to SrcName because we have Src's everywhere.

EDIT: VisName (yes short for visible), SurfaceName?