HPCC-10475 Avoid u8'string' going via a unicode type

Currently the Unicode type (incorrectly) counts lengths in codepoints
rather than abstract characters.  Avoiding going via Unicode type allows
utf8 constants to be treated correctly - without the upheaval of changing
the utf16 handling.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>