Mini-TT's utilities

This is a crate extracted from Mini-TT to help the development of other dependently-typed lambda calculus type checkers' command line interface.

It contains helper functions for the Clap (structopt) command line processor, file IO, and REPL helpers (for rustyline).

As I don't want to break the self-containing property of the Mini-TT codebase, I only extracted things from the CLI helpers, not the type-checker.