Remove calls to impure mathlib functions.
Implement pure floor()/ceil().
Implement pure tan().
Implement pure atan().
Implement pure atan2().
Implement pure exp().
Implement pure expm1().
Implement pure exp2().
Implement pure log().
Implement pure log10().
Implement pure log1p().
Implement pure log2().
Implement pure lrint().
Implement pure pow().
Fixup std.math unittests for new math implementations.
Merge pull request #1426 from ibuclaw/puremath

True pure std.math