The helper module bignum.py in the functional test framework provides only one function that is used outside the module (`bn2vch`) and it is used only in script.py, Remove `bignum.py` and move the function to `script.py`
PR17319 introduced a bug, so I had to merge also a commit from PR18378 which rewrites entirely the function `bn2vch`
This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/17319 | PR17319]]
Bugfix commit merged in the changeset:
https://github.com/bitcoin/bitcoin/pull/18378/commits/a3ad6459b77e9b7156e486a9e4a37dbfdc35e324
The rest of PR18378 will be in a separate diff.