Fix parse error for IPv4-mapped IPv6 addresses

[drvirgilio]

This is a simple fix for a parse error which isn't compliant with the specification. The issue is that h16 partially matches an IPv4 address but cannot backtrack when it fails.

> dhall format <<< 'https://[::192.168.0.1]/file'

Error: Invalid input

(stdin):1:14:
  |
1 | https://[::192.168.0.1]/file
  |              ^
unexpected '.'
expecting ']'

[Gabriella439]

Thank you! 🙂