Makes Int an opaque struct

[zslayton]

The Int enum has been renamed to IntData and is now pub(crate).
A new opaque struct type called Int now wraps it.

Part of issue #507, related to merged PR #568.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Patch coverage: 80.89% and project coverage change: +0.04 🎉

Comparison is base (c76655a) 83.23% compared to head (91fe9f9) 83.27%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #571      +/-   ##
==========================================
+ Coverage   83.23%   83.27%   +0.04%     
==========================================
  Files         104      104              
  Lines       19611    19628      +17     
  Branches    19611    19628      +17     
==========================================
+ Hits        16323    16346      +23     
+ Misses       1750     1745       -5     
+ Partials     1538     1537       -1     

Impacted Files	Coverage Δ
src/binary/non_blocking/raw_binary_reader.rs	80.91% <0.00%> (ø)
src/binary/uint.rs	92.30% <0.00%> (ø)
src/lazy/binary/raw/lazy_raw_value.rs	82.15% <ø> (ø)
src/text/parsers/containers.rs	97.53% <ø> (ø)
src/text/parsers/top_level.rs	92.20% <ø> (ø)
src/binary/raw_binary_writer.rs	85.33% <44.44%> (-0.07%)	⬇️
src/types/integer.rs	78.46% <76.08%> (+2.95%)	⬆️
src/lazy/binary/immutable_buffer.rs	87.02% <90.90%> (ø)
src/binary/int.rs	83.03% <92.85%> (+0.07%)	⬆️
src/binary/non_blocking/binary_buffer.rs	94.08% <100.00%> (ø)
... and 10 more

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