Second (smaller) set of native core functions

This rounds out the first pass at a set of core functions, adding
some that were not in the first PR.

From here I'm working on a contrib.seq and contrib.io module to
hold less obvious but maybe interesting native functions that can
move to core if desired.

This should also close out issure #150 asking for some core
functions like these.
slightly cleaner version of drop-while, could use yield-from when ready
One more commit to change the test for Python 2/3 to a macro

Note that this can't use the macro in hy.core.macros so there
is a local copy in language.hy