use 32-bit ints for values, add NaN tagging

[ffwff]

fixes #18

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (haru@ac5d6b8). Click here to learn what that means.
The diff coverage is 80.67%.

@@           Coverage Diff           @@
##             haru      #21   +/-   ##
=======================================
  Coverage        ?   84.62%           
=======================================
  Files           ?       32           
  Lines           ?     2816           
  Branches        ?        0           
=======================================
  Hits            ?     2383           
  Misses          ?      433           
  Partials        ?        0

Impacted Files	Coverage Δ
src/hanayo/time.rs	0% <0%> (ø)
src/vmbindings/function.rs	100% <100%> (ø)
src/vmbindings/value.rs	94.44% <100%> (ø)
tests/interpreter.rs	95% <100%> (ø)
src/vm/value.h	100% <100%> (ø)
src/hanayo/proc.rs	71.66% <100%> (ø)
tests/hanayo.rs	99.63% <100%> (ø)
src/vmbindings/gc.rs	83.7% <100%> (ø)
src/vmbindings/record.rs	95.65% <100%> (ø)
src/hanayo/int.rs	96.55% <100%> (ø)
... and 9 more

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[ffwff]

I've decided not to merge this due to a significant performance regression (benchmarks are 10-15% slower, however it consumes 10% less memory)

If anyone can find a way to fix this regression, please comment here.