This commit:
- Sorts the #include directives: The #includes of the ""-form come
first, those of the <>-form come later (this helps checking if any
include file is not self-sufficient); in each of the two groups, the
names of the headers or source files are sorted alphabetically (this
eases searching for a specific #include and avoids duplicates).
- Uses the ""-form when including Boost files; using the <>-form is a
relic of the past. This was even discussed on the developers list many
years ago and led to core issue 370. Note that some parts of
DynamicBitset were already using the ""-form and that has never caused
any problem, AFAIK.
- Removes some comments attached to the directives themselves which
seemed pretty useless and prone to get out of sync.
