MAINT: remove a bit of code duplication; remove import numpy, math

Lift moment, _munp, __call__ and freeze to rv_generic.
MAINT: Lift entropy to rv_generic.

rv_continuous version had a guard for vecentropy w/ numargs ==0, kept it.
MAINT: Lift the stats method to rv_generic.

Changed betwen rv_continuous and rv_discrete were:
* rv_c had guards for nan**1.5; kept.
* rv_c had a shortcut for no valid arguments; kept.
MAINT: lift generic _ppf and friends to rv_generic

Also trivial renamings: _veccdf vs _cdfvec etc.
MAINT: lift _argcheck to rv_generic.

rv_discrete(rv_continuous) version used bitwise(logical) and.
Kept the logical_and.
TST: reenable a dlaplace chi-square test; add tests for the edges of support
DOC: typos in the discrete logsf docstring
DOC: correct the docstring of _construct_argparser
MAINT: remove inspect calls from regular code paths (.stats & .moment)
Merge pull request #2724 from EvgeniBurovski/stats_refactor

refactor stats/distributions.py