For ease of use there should be a function telling the size of string for bignum_to_string()

Otherwise the user of ```bignum_to_string()``` should guess how many characters might be needed?

Start by locating bignum_to_string() and its declaration, then inspect how callers currently determine the output buffer size. Define the requested size-reporting behavior and verify it against representative bignum conversions, using any existing project checks available to confirm that callers can size the buffer correctly.

Written by the indexing model from the issue text.