checks: Refactor check output parsing

The check output/perfdata parsing is needlessly contrived (and
inefficient, though that is secondary.) This patch adds a cleaner
alternative interface to output parsing, makes better use of available
data structures and, presumably, improves performance. It also (imo)
makes the code a bit easier to follow.

Both ABI and API are preserved, through the parse_check_output function.

I've added tests as well, though those are not currently runnable, as
per GH issue #12.

Signed-off-by: Anton Lofgren <alofgren@op5.com>