Implement fast path for ASCII range in strnxfrm_onelevel_internal()

During benchmarking, Axel revealed a bottleneck inside MY_FUNCTION_NAME(strnxfrm_onelevel_internal) in ctype-uca.ic.

The problem is that calling my_uca_scanner_init_any() followed by a loop of MY_FUNCTION_NAME(scanner_next)() calls is costly.

Under terms of this task we'll add a fast path for the ASCII range into this strnxfrm_onelevel_internal().

• In the best scenario, when the input string is a pure ASCII, the execution will not even reach my_uca_scanner_init_any().
• In a worse scenario, when there is an ASCII prefix followed by a non-ASCII suffix, the ASCII part will be processed faster.