fix type-instability of logabsbeta

[stevengj]

Fixes #331.

Also omit an Int conversion for iseven in Julia ≥ 1.7 (JuliaLang/julia#38976)

[codecov]

Codecov Report

Merging #332 (06ae64b) into master (5f0a078) will decrease coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
- Coverage   92.72%   92.46%   -0.27%     
==========================================
  Files          12       12              
  Lines        2653     2653              
==========================================
- Hits         2460     2453       -7     
- Misses        193      200       +7     

Flag	Coverage Δ
unittests	92.46% <100.00%> (-0.27%)	⬇️

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Impacted Files	Coverage Δ
src/gamma.jl	94.91% <100.00%> (ø)
src/beta_inc.jl	89.45% <0.00%> (-1.09%)	⬇️

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