Use functions instead of the intrinsics

[gbaraldi]

Intrinsics aren't promised to always be there, specially because some functions can be moved from intrinsics to functions.
See JuliaLang/julia#48685. The functions are probably more stable so use them by default

[codecov-commenter]

Codecov Report

Merging #617 (6bf3b43) into master (18e1ed1) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 6bf3b43 differs from pull request most recent head 8532249. Consider uploading reports for the commit 8532249 to get more accurate results

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@@           Coverage Diff           @@
##           master     #617   +/-   ##
=======================================
  Coverage   89.05%   89.05%           
=======================================
  Files          16       16           
  Lines        1489     1489           
=======================================
  Hits         1326     1326           
  Misses        163      163           

Impacted Files	Coverage Δ
src/fastmath.jl	87.14% <100.00%> (ø)

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