Added `as_string_8_conversion' and `as_string_32_conversion' to help users migrating by marking places in their code where the STRING_8 <-> STRING_32 conversions needs to be worked on at a later time.
See review#9175108.


git-svn-id: https://freeelks.svn.sourceforge.net/svnroot/freeelks/trunk@697 6d4a7a74-6026-0410-acef-abb7cea3cd6d
Removed implicit conversion STRING_32 -> IMMUTABLE_STRING_32.


git-svn-id: https://freeelks.svn.sourceforge.net/svnroot/freeelks/trunk@698 6d4a7a74-6026-0410-acef-abb7cea3cd6d