Use CRlibm 0.5 that does not export functions

[dpsanders]

CRlibm 0.5 (the latest release) no longer exports functions, so we define them explicitly in rounding.jl.

This is in preparation for #220, but should be included in v0.7.

[codecov-io]

Codecov Report

Merging #238 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
+ Coverage   91.85%   91.86%   +<.01%     
==========================================
  Files          26       26              
  Lines        1044     1045       +1     
==========================================
+ Hits          959      960       +1     
  Misses         85       85

Impacted Files	Coverage Δ
src/intervals/rounding.jl	76.47% <100%> (+1.47%)	✅

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[coveralls]

Coverage increased (+0.008%) to 91.866% when pulling b28602a on use_crlibm_0_5 into a24a7f3 on master.

[coveralls]

Coverage increased (+0.008%) to 91.866% when pulling b28602a on use_crlibm_0_5 into a24a7f3 on master.