Improve random float generation for test cases

[alexcrichton]

Float generation happens here, and #165 is an example where we could, for example, generate values near pi for edge case testing of trig functions.

[tgross35]

In the past few months I have completely reworked tests - we now have dedicated tests for edge cases and run exhaustive tests, both against MPFR. So I think the situation is much improved here :)