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 :tada:

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

:exclamation: 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

```diff @@ 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](https://codecov.io/gh/libsv/go-bt/pull/150?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libsv) | Coverage Δ | | |---|---|---| | [tx.go](https://codecov.io/gh/libsv/go-bt/pull/150?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libsv#diff-dHguZ28=) | `76.45% <90.19%> (+0.37%)` | :arrow_up: | | [input.go](https://codecov.io/gh/libsv/go-bt/pull/150?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libsv#diff-aW5wdXQuZ28=) | `82.30% <100.00%> (+2.70%)` | :arrow_up: | | [output.go](https://codecov.io/gh/libsv/go-bt/pull/150?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libsv#diff-b3V0cHV0Lmdv) | `90.00% <100.00%> (+3.04%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://codecov.io/gh/libsv/go-bt/pull/150/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libsv) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libsv). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libsv)

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[mergify[bot]]

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