Add Julia translation of psifn from SLATEC and some unicode constants

This request is related to #3226 

Right now I use i1mach and d1mach from opemlibm-extras since they were used in the SLATEC original but is there a more Julian way of getting those numbers? Also, any thoughts on the error messages. Right now most of them are taken from the source.

I allowed myself to introduce the Euler gamma for normal floats because it was already exported for BogFloats. I also introduced the unicode aliases π and γ. It has been discussed at some point and I like it.