Bigint fixes from https://github.com/hebisch/poplog need incorporating

[sfkleach]

Waldek Hebisch has added a series of fixes for bigint arithmetic that need to be mirrored in GetPoplog. These come from his github repo.

• Fix bigint left shift of negative numbers
• Fix returning pop integers from bigint functions
• Do not use uninitialized memory during bigint divide
  • Use safer way to return result of bigint multiplication
  • Discover more cases of 0 quotient in bigint divide