Use UV instead of Size_t.

A strong reason for using UV are the promised custom codepoints,
they go beyond 32-bit.

The overflow logic didn't work if Size_t was 32-bit but the UV was 64-bit.
Steal the battle-proven logic from grok_number_flags().

The numeric.xs or grok.t were not right in 32-bit, either.

Add comments.
Share common constants as file statics.