Use mpfr_lngamma for loggamma(::BigFloat)

[devmotion]

This PR uses mpfr_lngamma for the implementation of loggamma(::BigFloat). Currently, loggamma(::BigFloat) is computed via logabsgamma(::BigFloat).

[codecov]

Codecov Report

Merging #351 (35c1cdd) into master (0c41812) will increase coverage by 0.31%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
+ Coverage   92.75%   93.06%   +0.31%     
==========================================
  Files          12       12              
  Lines        2706     2712       +6     
==========================================
+ Hits         2510     2524      +14     
+ Misses        196      188       -8     

Flag	Coverage Δ
unittests	93.06% <100.00%> (+0.31%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/gamma.jl	94.90% <100.00%> (+0.08%)	⬆️
src/beta_inc.jl	91.87% <0.00%> (+1.20%)	⬆️

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