Add a few more test cases for scanning (integer conversion),
which were helpful in debugging some upcoming changes.
BenchmarkScan currently times converting the value 10**N
represented in base B back into []Word form.
When B = 10, the text is 1 followed by many zeros, which
could hit a "multiply by zero" special case when processing
many digit chunks, misrepresenting the actual time required
depending on whether that case is optimized.
Change the benchmark to use 9**N, which is about as big and
will not cause runs of zeros in any of the tested bases.
The benchmark comparison below is not showing faster code,
since of course the code is not changing at all here. Instead,
it is showing that the new benchmark work is roughly the same
size as the old benchmark work.