Move Fibrations.v out of Basics & add stuff using Types.Sigma
add more documentation for `simpl rewrite`
Merge pull request #587 from mikeshulman/fibrations

Move Fibrations.v out of Basics & add stuff using Types.Sigma
Merge pull request #588 from mikeshulman/rewritedoc

add more documentation for `simpl rewrite`
./autogen.sh