Get rid of shallow_repr and print full terms

[yannham]

For a long time, we've used shallow_repr() as a light pretty printer for terms in error messages. The issue is that this printer is very uninformative: anything past constant is printer as { ... } or [ ... ]. It dates back to times where we didn't have a proper pretty printer, but now we have (and we can cap the output if we don't want the message to be too long).

This PR get rid of shallow_repr and use the Display instance for RichTerm instead. It has the drawbacks of sometimes exposing generated variables (and, very soon, will be closure addresses) - we might want, in the future, to substitute some of the environment variables to get a better output. Still, even with generated variables, we have much more information about the structure of a term in an error message (field names, array length, etc.). Most of the error messages use codespan to print those terms, and codespan is handling long input on its own (only showing the beginning of the snippet and the end). Elsewhere, we used pretty_print_cap.