Use IrrationalConstants and LogExpFunctions.log1mexp

[devmotion]

This PR uses IrrationalConstants, which is already an indirect dependency of SpecialFunctions through LogExpFunctions 0.3, for values such as sqrt(pi) or pi/2 and removes the definition of log1mexp in favour of LogExpFunctions.log1mexp.

Moreover, I noticed three occurrences of log(1-...) and replaced them with log1p(-...).

[codecov]

Codecov Report

Merging #345 (5963cf1) into master (d4f6d7e) will decrease coverage by 0.45%.
The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master     #345      +/-   ##
==========================================
- Coverage   92.90%   92.45%   -0.46%     
==========================================
  Files          12       12              
  Lines        2722     2717       -5     
==========================================
- Hits         2529     2512      -17     
- Misses        193      205      +12     

Flag	Coverage Δ
unittests	92.45% <88.46%> (-0.46%)	⬇️

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Impacted Files	Coverage Δ
src/SpecialFunctions.jl	100.00% <ø> (ø)
src/gamma_inc.jl	92.62% <66.66%> (ø)
src/gamma.jl	94.55% <80.00%> (-0.03%)	⬇️
src/beta_inc.jl	89.77% <100.00%> (-1.81%)	⬇️
src/betanc.jl	90.19% <100.00%> (ø)
src/ellip.jl	100.00% <100.00%> (ø)
src/erf.jl	97.95% <100.00%> (-0.05%)	⬇️

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[stevengj]

(Aren't a bunch of these constants also used in chainrules.jl?)