some cleanup and checks for structured schemas

Summary:
moved options, key types, and the max key length into constants to avoid typos.
Added check for key values at schema, table, and column levels.

Test Plan: go test ./structured/...

Reviewers: spencerkimball

Reviewed By: spencerkimball

Subscribers: team

Differential Revision: http://phabricator.cockroachdb.org/D84