write_bigint_bytes: strip leading 0s (and also sometimes leading 255s)

[est31]

This stripping is required, some parsers don't like these leading numbers.

[est31]

Quoting X.690 here, section 8.3.2:

If the contents octets of an integer value encoding consist of more than one octet, then the bits of the first octet
and bit 8 of the second octet:

• a) shall not all be ones; and
• b) shall not all be zero

For the check to remove leading 0 bytes, we don't look at the highest bit of the next octet because otherwise, if that bit is one, we'd not remove the leading 0 and confuse the entire value with a 0 further below. There is no problem in not checking the highest bit because even if it is 1, and the removal was "illegal", the code below re-adds a leading 0.

For the check to remove leading 255 bytes, we only do it if the positive flag is unset; otherwise we'd get into trouble in situations where we have removed any leading 0 but were going to add it back in code below: [0, 255, 255] is supposed to be a fixpoint as that encodes 65535. We do the highest bit checking there because otherwise we might remove a leading 255 when we shouldn't.