Enable Numbers of length() greater than base_.

[matt-gretton-dann]

The bc length() function returns the length of a number in terms of the number of digits.

The current implementation assumes that this will fit in NumType and so be less than the base of
numbers we are storing. This is surprisingly limiting (esp. if NumType is 8- or 16-bits).

We should update this so that the maximum length is either:

• base_ * base_log10_: That is you can have at most base_ underlying digits in the number
  array; or
• std::vector<>::max_size() * base_log10_: You can have Numbers as long as you like.

Assignees: @matt-gretton-dann
Labels: bc