Use `nyan-interpolation`

[dcastro]

Clarification and motivation

In xrefcheck, we often need to build and report error messages.

So far, we've used a mix of string concatenation, manually adding \n, unlinesF, indentF, mconcat, etc.

Let's try a cleaner and more unified approach with an interpolation library like nyan-interpolation.

Acceptance criteria

• We're using nyan-interpolation to build error messages.