113-bit integer arithmetic

Changes for 0.04

• Allow negative values to be passed to bitwise operators (&, |, ^, ~, >>, <<). These negative values will be converted to their 113-bit 2s-complement value prior to being acted upon by the bitwise operator - thus mimicking perl's treatment of negative 32-bit/64-bit IVs.
• Add divmod() function.
• Add new test file t/bitwise_feature.t