Extensive MODEXP testing WIP

[OlivierBBB]

We need to write tests for MODEXP that will test the various combinations of the following:

Tiny call data: have a `CDS` parameter of
0,
19, 32 + 19, 64 + 19,
32 - 1, 64 - 1, 96 - 1, 

// 19 is just some fixed value

32 * x + 19 cases: make it so that the call data contains NONZERO bytes after the last call data byte
32 * x - 1  cases: make it so that the final byte is 0x02 and the following bytes are random nonzero values

The purpose of the "-1" case is to test for the legal value 0x200 = 512 of one of the bbs, ebs, mbs values