[tests] remove bignum.py

Summary:
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 PR17319

Additional bugfix commit:
https://github.com/bitcoin/bitcoin/pull/18378/commits/a3ad6459b77e9b7156e486a9e4a37dbfdc35e324

The rest of PR18378 will be in a separate diff.

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8852