Fix issue #2969: correct guess for number of series terms of f(x) for (f(x))**a
order.py: implement simplifications of powers (e.g. O(x**sqrt(-x)) -> O(x**(S(3)/2)))
merge smichr's changes as-is
Inline _omatch() code + some simplifications
remove duplicate symbols + some cleanup by smichr
Correct nuse in e683d5b
Add @XFAIL test for issue 3683
Merge pull request #1863 from skirpichev/2969-Power-of-series

Fix for issue 2969