Fix f32 and f64

[therustmonk]

The crate doesn't always handle floats correctly. For example it can panic for NaN value in From<f32> implementation, but later it wrapped with Some(_) that will never work.

This PR fixes how the library works with floats and offers TryFrom implementation instead of From.

Also I've fixed some deprecation warnings.

[codecov-commenter]

Codecov Report

Merging #65 into master will decrease coverage by 0.35%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
- Coverage   95.20%   94.84%   -0.36%     
==========================================
  Files           2        2              
  Lines        1438     1436       -2     
==========================================
- Hits         1369     1362       -7     
- Misses         69       74       +5     

Impacted Files	Coverage Δ
src/macros.rs	80.00% <ø> (ø)
src/lib.rs	95.00% <100.00%> (-0.36%)	⬇️

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[vlmutolo]

Just want to say thanks for taking this on! There's actually a bug in Nushell that results from problems that this PR fixes. I was trying to figure out how to make these changes myself when I found your patch.

[akubera]

Thanks. I was trying to avoid excluding versions of rust supported by the num-bigint crate (currently limited to 1.31), but I think TryFrom (1.34 -- from over a year ago!) is a good enough reason to bump the min-version.