Include needed parts at place where they are needed instead of including the
whole Proto library and significant parts of Phoenix in a few places.
Parsing time reported by Clang 9 with default (C++14) std:
was |now |gain|include
----|----|----|----------
4.45|4.22|5.2%|qi
4.52|4.23|7.5%|karma
4.37|3.99|9.7%|lex
3.75|3.47|7.5%|qi_numeric
There were phoenix/limits.hpp inclusion before every Proto inclusion to override
Proto limits with Phoenix limits values. It has no value in C++11+ world, and
poisons code too much as the number of places where Proto headers now included
increased, so I did not replicate it.
