Enhancement: NewTxFromReader and VarInt datatype

[tigh-latte]

Add NewTxFromReader that builds and returns a transaction from an io.Reader

Also added VarInt data type for cleaner handling of variable ints.

[codecov-commenter]

Codecov Report

Merging #88 (1857737) into master (792a281) will decrease coverage by 0.72%.
The diff coverage is 72.25%.

❗ Current head 1857737 differs from pull request most recent head d9d468a. Consider uploading reports for the commit d9d468a to get more accurate results

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
- Coverage   84.51%   83.78%   -0.73%     
==========================================
  Files          35       35              
  Lines        3512     3627     +115     
==========================================
+ Hits         2968     3039      +71     
- Misses        396      424      +28     
- Partials      148      164      +16     

Impacted Files	Coverage Δ
varint.go	73.77% <65.85%> (-26.23%)	⬇️
input.go	81.08% <71.42%> (-9.17%)	⬇️
tx.go	78.76% <72.91%> (-1.89%)	⬇️
output.go	86.95% <73.91%> (-13.05%)	⬇️
signaturehash.go	70.86% <100.00%> (ø)
txchange.go	82.75% <100.00%> (ø)
txinput.go	85.26% <100.00%> (ø)
txoutput.go	82.50% <100.00%> (ø)

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[theflyingcodr]

LGTM