Display the full type environment when reporting type holes

This fixes Trac #8191.

The patch also adds and documents a new flag -fmax-relevant-bindings=N
which lets you control how many bindings in the type environment are shown.
Improve debug error message for applyTypeToArgs
Improve TcSimplify.approximateWC, fixing Trac #8155

See Note [ApproximateWC]
Refactor to avoid gratuitous DEBUG warning
Improve docs for -XNegativeLiterals