Followup: https://github.com/ruby/strscan/pull/115

`scan_integer` is now implemented in Ruby as to efficiently handle
keyword arguments without allocating a Hash. Given the goal of
`scan_integer` is to more effciently parse integers without having to
allocate an intermediary object, using `rb_scan_args` would defeat the
purpose.

Additionally, the C implementation now uses `rb_isdigit` and
`rb_isxdigit`, because on Windows `isdigit` is locale dependent.