Small improvements to (a|u)strings.inc.

This patch:

astr.patch

Slightly improves several string functions.

fpc_AnsiStr_Unique saves two memory reads.

ansistr_unique

fpc_AnsiStr_Copy saves some code and branches.

ansistr_copy

fpc_AnsiStr_SetLength changes signed div 2 to unsigned.

ansistr_setlength

Same for UnicodeStr.

Edited by Rika