Optimise numeric division for 3 and 4 base-NBASE digit divisors.

Optimise numeric division for 3 and 4 base-NBASE digit divisors.

On platforms with 128-bit integer support, introduce a new function
div_var_int64(), along the same lines as div_var_int() added in
d1b307eef2 for divisors with 1 or 2 base-NBASE digits, and use it to
speed up div_var() and div_var_fast() in a similar way when the
divisor has 3 or 4 base-NBASE digits.

This gives significant performance gains for divisors with 9-16
decimal digits.

Joel Jacobson.

Discussion:
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