Double is now f64, leveraging rust's IEEE-754 + optional feature "sets"

[Grinkers]

This is a continuation of #110.

This should be after #113. Not worth dealing with merge conflicts, I'll just rebase to keep the history clean.

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (36f9a1d) 71.27% compared to head (2932993) 71.38%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
+ Coverage   71.27%   71.38%   +0.10%     
==========================================
  Files           8        8              
  Lines         947      940       -7     
==========================================
- Hits          675      671       -4     
+ Misses        272      269       -3     

Files	Coverage Δ
src/deserialize/mod.rs	59.67% <100.00%> (+0.47%)	⬆️
src/deserialize/parse.rs	93.28% <ø> (ø)
src/edn/utils/index.rs	19.78% <ø> (ø)
src/json/mod.rs	100.00% <100.00%> (ø)
src/lib.rs	100.00% <ø> (ø)
src/edn/mod.rs	60.27% <75.00%> (-0.87%)	⬇️

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[Grinkers]

Rebased and pushed, while allowing all tests to pass. I think the implementation is ready for review, but I'm not too happy with how many #[cfg")] are scattered about. Some amount is unavoidable, but in particular the tests tests are a little ugly. I took a look at some other crates and the common pattern there seems to be something like

// some_tests.rs
mod generic_tests;
#[cfg(feature = "sets")]
mod tests_with_sets;
...

I wanted to take a look at Error handling and a borrow impl (less heap allocations, requiring a walker). I think that might be the time/place to fix/clean this up.
https://github.com/rust-embedded-community/serde-json-core/blob/9deb2b91ea737df79f33f7536587b59ed5f141ee/src/de/mod.rs#L23
for reference for what I sort of had in mind. These can now be added now that we have non_exhaustive