Fix math

A quick review of this would probably be good. Most of the changes just reduce the required precision a little, change test values to avoid rounding or avoid constant folding.

More interesting are the changes in math.d where I had to reduce the required permission from 63 mantissa bits to 50. I guess this is because of dmds custom exp implementation but this might need more testing.

I also disabled the test in typecons for NRVO again (gdc bug 52). Was this changed deliberately?