Adjust fixed_ring_buffer.read()

Returns an object from the buffer only if can_read is true, otherwise
just returns a default-initialized object.
Add fixed_ring_buffer.clear()
Add fixed_vector.clear()
Add fixed_ring_buffer.is_contiguous() and .make_contiguous()

Former returns whether the buffer's data is contiguous (i.e., doesn't
wrap around), latter forces it to be contiguous.
Add CAPACITY constant.
Mark member variables as private.
Add capacity member function.
Add an iterator type to the ring buffer.

The iterator permits random read-only access to the ring buffer's
contents. Currently, any operation on the ring buffer should be assumed
to invalidate iterators.
Basic doc comments for everything.