Optimize from/assign str for power of 2 bases

[OTheDev]

This leads to a very significant speedup for bases 2 (0b10), 4 (0b100), 8 (0b1000), 16 (0b10000), and 32 (0b100000).

This PR also does a few more things, which probably should have been done independently from this PR.

• Ensures that from/assign-str functions never preallocate more than one more than the number of Arbi::BASE digits actually needed to store the number the string represents. The initial calculation involving div_ceil() is great for small strings, but for very large strings, it may lead to an overallocation by hundreds or even thousands of Arbi::BASE digits. This applies for all valid bases [2, 36].
• Fixes a bug in size_base()-type functions that used > 0 instead of != 0 when dividing an Arbi integer in-place to determine the number of base-base digits. != 0 is needed, rather than > 0 , in case the Arbi integer is negative.