Add initial num-traits support

[tarcieri]

Adds a conditional impl of the Num trait for F32, gated on the num-traits feature.

Closes #57.

[codecov-commenter]

Codecov Report

Merging #80 (766f67f) into main (b7f6919) will decrease coverage by 0.73%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
- Coverage   61.04%   60.31%   -0.74%     
==========================================
  Files          38       38              
  Lines         824      834      +10     
==========================================
  Hits          503      503              
- Misses        321      331      +10     

Impacted Files	Coverage Δ
src/float.rs	51.79% <0.00%> (-4.02%)	⬇️

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