Adds additional zero checks for setCompact

This pull request adds additional checks to the setCompact() method for bignums.

It covers various valid representations of zero (positive and negative).  It tests that the non-zero portions of the encoding are correctly ignored and that they are re-encode back to all zeros.

   Positive Numbers
    0x01003456 (1 byte  0x00)
    0x02000056 (2 bytes 0x0000)
    0x03000000 (3 bytes 0x000000)
    0x04000000 (4 bytes 0x000000_00)

   Negative Numbers
    0x00923456 -(0 bytes, so zero)
    0x01803456 -(1 byte  0x00)
    0x02800056 -(2 bytes 0x0000)
    0x03800000 -(3 bytes 0x000000)
    0x04800000 -(4 bytes 0x000000_00)