refactor top_down, bottom_up to use strategic all
top_down_once, bottom_up_once
do_one branching rule
move ident to strat_pure
branching strategic all - refactor top_down
add do_one and identity imports
rename rules core
rename strat/branch core
remove strat.py, fix import in test_core
rename rules module to strategies
rules -> strategies in matrix expressions
rules -> strategies in unify
rules -> strategies in setup.py
Merge pull request #1804 from mrocklin/strat-refactor

Refactor strategies