Improve error message display

[yav]

It has become pretty common for compiler to show some context and underline the location of the error in error messages. Cryptol is pretty good about keeping track of the locations of things, so it should be fairly simple to also show this.

Here's an example of the kind of thing we want:

Cryptol> `Integer

[error] at <interactive>:1:2--1:9:
  Incorrect type form.
    Expected: a numeric type
    Inferred: a value type
|
| `Integer
|  ^^^^^^^
Cryptol>