common: bitset_set and mini_flat_map

The bitset_set is compile-time-fixed size bitmap, which can be accessed using a std::set-like iterator.

The mini_flat_map is similar to a boost::flat_map, except with more restrictions on the size and key, allowing it to make
better use of vectors.

Both have the restriction that the key must be unambiguously castable to/from a small integer. The mini_flat_map has the additional restriction, that the size must be known at construction time.

Signed-off-by: Alex Ainscow <[email protected]>

Author: aainscow