Investigate precision of invIncompleteBeta a b x for very small x

[Shimuuar]

Function could return answers which are orders of magnitude off. It could be due to the fact that we revert to bisection which is linear instead of cubic Halley.

>>> (\a b -> incompleteBeta a b . invIncompleteBeta a b) 90 90 1e-200
9.866213992257607e-197

[Shimuuar]

Things are much worse:

*Statistics.Distribution.StudentT> invIncompleteBeta 0.5e-2 0.5 0.84013
8.881784197001252e-16
*Statistics.Distribution.StudentT> invIncompleteBeta 0.5e-2 0.5 0.84012
0.0