Fix shl, shr & simplify isZero

[konsumlamm]

Now a: BigInt is zero if and only if a.limbs is empty. This simplifies isZero and reduces special cases to consider when consuming a BigInt.

While checking that all functions uphold this invariant, I noticed that shl and shr were buggy:

• shl didn't check for zero, which allowed the user to construct BigInts with arbitrarily many zeros in its limbs, breaking an invariant
• shr didn't check if the shift is too big, which could result in an exception

[konsumlamm]

Division is making too much problems, perhaps I'll reconsider this once division has been rewritten.

[dlesnoff]

If you have some failing checks, maybe this could be used for fixing the division algorithm. Do not hesitate to share these checks on the #123 issue.

[konsumlamm]

The problems are that unsignedDivRem relies on 0 having a limb in a lot of places and trying to replace those cases manually is a pain. Some of these cases should go away once we have operations with SomeInteger operands, but eventually it should probably get rewritten.