Tricomi U small argument

To fully answer [this comment](https://github.com/JuliaMath/HypergeometricFunctions.jl/commit/2b47e2f3cdad38d54154fc06e35bfb25ac16f7a3#commitcomment-62643489), it takes [Kummer's transformation](https://dlmf.nist.gov/13.2#E42) for U and for integer `b` will require the limiting form of [13.2.9](https://dlmf.nist.gov/13.2#E9). In the vicinity of an integer `b`, probably the method of [Michel and Stoitsov](https://doi.org/10.1016/j.cpc.2007.11.007) for Gauss' function would be useful.