Clean up POD for Math::BigInt and Math::BigFloat

Based on work done by Peter John Acklam, differences from his original
patch:

- links to items (which have been corrected by others) have been retained

- unnecessary conversion of < and > to E<lt> and E<gt> has been skipped.

- =over N has been converted to =over

- adjusted white-space in many places to avoid hitting column 80 in
  perldoc for verbatim text
bump versions for Math::BigInt and Math::BigFloat