change _evalf to _evalf_binprec to make difference obvious
name helper evalf as _evalf
allow .evalf to take None for n; unify evalf arglist names
raise TypeError if subs for evalf is not a dictionary
define n when you define evalf (BasisDependent)
is_zero and free_symbols modifications

is_zero is needed to identify alternate *trivial* forms of 0 as
in Integral(0, ...). It should never do more than look for a literal
0 someplace. For this reason, the Integral.is_zero routine was modified
and the advanced processing for other mostly trivial cases of an integral
being 0 was moved to the doit method.

foo.is_number is equivalent to not foo.free_symbols but is a faster way of
arriving at that answer because is_number will fail if a single free
symbol is encountered. For this reason, the Integral.is_number routine
was modified.

foo.free_symbols should give any free symbols in the expression *as
written* without any regard to whether those symbols will disappear after
simplification of some sort. The test for is_zero was removed from
expr_with_limits calculation of free symbols for this reason.

Other comments about the rationale of these changes are in the test
suite (esp in test_basic) so that anyone changing the behavior will be
sure to see the desired state of affairs.
add check for is_zero in re and im
add literal flag to evalf
modify is_comparable
use literal flag in assumption related to numbers
7642: KroneckerDelta more judicious in evalf; = 0 for nonzero diff
make is_positive and is_negative like new assumptions
modify test which takes a long time