Document Deque.
Move in-place functions below their iterator variants.
Place union as the first function, for consistency.
Group union, intersect and difference in Bitv.
Document Bitv.
Document BitvSet.
Move intersection above difference and symmetric_differance.

So all comes in the order union, intersection, difference and
symmetric_difference.
Main bitv example: prime sieve.
Enclose None as `None`.
Polish bitv docs.
Describe BitPositions and TwoBitPositions.
Simplify and cleanup bitv examples.
auto merge of #15824 : treeman/rust/deque-bitv-doc, r=huonw

Examples for `Deque`, `Bitv` and `BitvSet`. Normalize documentation style and some source code reorganization.