[wpiutil] Add reverse iterators to wpi::circular_buffer and wpi::static_circular_buffer, make other iterators bidirectional

[petelilley]

Changes to circular_buffer and static_circular_buffer:

• Made iterator and const_iterator bidirectional
• Added reverse_iterator and const_reverse_iterator using std::reverse_iterator<>
• Added relevant rbegin(), rend(), crbegin(), crend() functions
• Added unit tests for both iterators

I also added comments to circular_buffer to match the ones in static_circular_buffer

[calcmogul]

The const iterator getters are inconsistent about whether they say they return a "const iterator" or not. It's probably best to include the const where applicable.

[petelilley]

Comments are updated