Added docstring to lambdified functions

If arguments have names, that is used. Otherwise the stack is inspected
to determine if the name can be inferred *with certainty*. If there is
more than one variable pointing to the object, then the name cannot be
determined with certainty, and 'arg_#', where '#' is the number of the
arg is used.
Merge remote-tracking branch 'upstream/master' into lambdify_docstring

Conflicts:
	sympy/utilities/lambdify.py
Added expression printing to lambdify docstring

If the expression is less than 78 characters, the whole expression is
printed. Otherwise, the first 75 characters are printed, followed by
'...'.
Merge pull request #8003 from jcrist/lambdify_docstring

Added docstring to lambdified functions