Always return longs from decoded integers

[greglook]

Fixes #4.

[codecov-io]

Codecov Report

Merging #5 into develop will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           develop       #5   +/-   ##
========================================
  Coverage    99.86%   99.86%           
========================================
  Files           13       13           
  Lines          730      730           
  Branches         1        1           
========================================
  Hits           729      729           
  Partials         1        1

Impacted Files	Coverage Δ
src/clj_cbor/header.clj	100% <100%> (ø)	⬆️
src/clj_cbor/codec.clj	99.67% <100%> (ø)	⬆️

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