Malloc and performance optimisations for Clone TX and toBytesHelper

[icellan]

Malloc is expensive and this pull request removes all mallocs from toBytesHelper used by ToBytes() and ToExtendedBytes(). This makes it much faster to get the bytes of a transaction when you are doing millions of transactions per second.

I also optimized Clone() of tx, which is used in various places, for instance when validating signatures.

[codecov-commenter]

Codecov Report

Patch coverage: 94.44% and project coverage change: +0.02 🎉

Comparison is base (79b6991) 80.61% compared to head (1999018) 80.64%.

❗ Current head 1999018 differs from pull request most recent head 8281ffe. Consider uploading reports for the commit 8281ffe to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
+ Coverage   80.61%   80.64%   +0.02%     
==========================================
  Files          38       38              
  Lines        4112     4179      +67     
==========================================
+ Hits         3315     3370      +55     
- Misses        546      559      +13     
+ Partials      251      250       -1     

Impacted Files	Coverage Δ
tx.go	76.45% <90.19%> (+0.37%)	⬆️
input.go	82.30% <100.00%> (+2.70%)	⬆️
output.go	90.00% <100.00%> (+3.04%)	⬆️

... and 1 file with indirect coverage changes

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[mergify]

This pull request looks stale. Feel free to reopen it if you think it's a mistake.