Added `append_substring' and `prepend_substring' for faster insertions in STRING_GENERAL and provided STRING_8 and STRING_32 efficient implementations.
See review#9270049.


git-svn-id: https://freeelks.svn.sourceforge.net/svnroot/freeelks/trunk@733 6d4a7a74-6026-0410-acef-abb7cea3cd6d