math.big: optimize divide_array_by_digit()

[tankf33der]

Capacity for quotient array already defined in integer.v like this:

...
       // Division for positive integers
        mut q := []u32{cap: int_max(1, dividend.digits.len - divisor.digits.len + 1)}
        mut r := []u32{cap: dividend.digits.len}
        divide_digit_array(dividend.digits, divisor.digits, mut q, mut r)
...

so just use it instead of full operand_a.
One of the important clients of this function is str() function.
Everyone will feel the difference.

Performance improvements:

• "./v test vlib/math/big/" works ~3+ times faster
• str() on ~180k digits works ~6 times faster

[huly-for-github]

Connected to Huly®: V_0.6-22932

[enghitalo]

Good catch!

[spytheman]

Excellent.