Move Matchers from github.com/prometheus/prometheus/model/labels to common

[siavashs]

Prometheus has Matcher and FastRegexMatcher.

Alertmanager currently uses an old fork of the label Matcher prometheus/alertmanager#2009

So I'm suggesting we move these matchers to common and point both Prometheus and Alertmanager to common.
Alertmanager can especially benefit from the new FastRegexMatcher.