Unwind back to somewhere in the tree we can make progress from
Speed up resolution by considering exact reqs first

Exact requirements keep the tree constraint and thus speed things up by
restricting the ammount of backtracking that is available.
Add stats to the Resolver to help understand it's performance
Merge branch 'fix-dep-res'

Conflicts:
	lib/rubygems/resolver/requirement_list.rb
Fix a few failing test caused by fixed dep resolver