round out lexicographical ordering tests in key encodings

Summary:
all that's left is working out the float precision issues and testing ordering for those as well.

towards a fix for #31

Test Plan: unit tests

Reviewers: petermattis, spencerkimball

Reviewed By: petermattis, spencerkimball

Subscribers: team

Differential Revision: http://phabricator.cockroachdb.org/D133