refine int64 handling

[gfx]

To use DataView heavily and reduce the code.

[codecov-io]

Codecov Report

Merging #19 into master will increase coverage by 0.28%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
+ Coverage      95%   95.28%   +0.28%     
==========================================
  Files          12       12              
  Lines         660      679      +19     
  Branches      143      150       +7     
==========================================
+ Hits          627      647      +20     
- Misses         12       13       +1     
+ Partials       21       19       -2

Impacted Files	Coverage Δ
src/Decoder.ts	99.59% <100%> (-0.02%)	⬇️
src/utils/int.ts	100% <100%> (ø)	⬆️
src/ExtensionCodec.ts	97.34% <100%> (+1.94%)	⬆️
src/Encoder.ts	95.1% <100%> (ø)	⬆️

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