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

This patch:

Slightly improves several string functions.

fpc_AnsiStr_Unique saves two memory reads.

fpc_AnsiStr_Copy saves some code and branches.

fpc_AnsiStr_SetLength changes signed div 2 to unsigned.

Same for UnicodeStr.

Edited Jul 26, 2023 by Rika

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