A series of Bigint arithmetic fixes copied from Waldek Heibsch, Closes #120

[sfkleach]

Waldek noticed a series issues in the bigint libraries and corrected them in his repository. This PR copies across the commit messages and content to ease comparison. A single minor whitespace change has been made from Waldek's commits but line numbers have been preserved to reduce divergence.

The original commits were:

• 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

A further commit has been added that resolved issues in the function testbit.

IMPORTANT NOTE: Ideally we will include unit tests with this PR but this is submitted now in order to begin the review process.

[sfkleach]

Build not working correctly but these changes are imported from a working system & verified locally