List available environment variables in help output

Added environment variables (declared with Variable in Options) to the console output if a user runs a program with --help.

Fixed the escaping of strings when serializing JSON. We hadn't noticed this for a long time, but some strange characters in Show instances tripped us up when sending telemetry to Honeycomb. Turned out to be improper escaping. Twice. It's fully conformant with the RFC spec now.

Added unconsRope (long overdue) and breakRope (which is really just a convenience wrapper around findIndexRope and splitRope). Made some related if minor performance and correctness improvements inside core-text.

What's Changed

• Output known environment variables by @istathar in #110
• Escape backslashes in addition to quotes by @istathar in #112
• Fix json escaping again by @istathar in #115
• Fix reading metadata from .cabal files by @istathar in #116

Full Changelog: core-telemetry-0.1.9.3...core-program-0.4.5.3