Add setBit, clearBit and testBit

[demotomohiro]

This PR adds setBit, clearBit and testBit procs. They are same to setBit, clearBit and testBit in std/bitops.

[konsumlamm]

I like this, however I'd prefer if this could treat negative numbers as if they were represented in 2's complement, like in #111. For that, if the input is negative, set isNegative to false, decrement it (this is done by invertIn in #111, although it copies the BigInt first), then treat every bit as inverted (and the other bits that aren't stored as 1, so clearBit would then need to update the length, instead of setBit) and keep in mind that the updated bit also needs to be inverted. Then, at the end, increment, and set isNegative to true again (this is done by invertOut in #111).