Pretty print error location

[sim642]

In Drup/zarith-ppx#1 (comment) came up the point that mdx doesn't pretty print error locations like the OCaml toplevel does.
As a very random example, here it uses carets to point to the location span:

# let x = 1.2z;;
Line 1, characters 8-12:
1 | let x = 1.2z;;
            ^^^^
Error: Unknown modifier 'z' for literal 1.2z

(this is with TERM=dumb though; by default underlining is used, but that won't work in Markdown code blocks).

Currently mdx doesn't quote nor pretty print the location, producing output like:

# let x = 1.2z;;
Line 1, characters 8-12:
Error: Unknown modifier 'z' for literal 1.2z

Quoting duplicates the (single-line) input, which might be too verbose, so such behavior might have to be optional.

[Leonidas-from-XIV]

It's definitely a nice feature. The only thing that might be problematic (I haven't tested it yet) is that different OCaml compilers display this differently, which MDX would need to unify, since nobody likes their tests to fail because the ^^^^ are now in a slightly different spot.

I feel like this should be reasonably easy to add if we get the location information from the compiler.

[sim642]

There's pp_loc, which might be more stable than compilers.