[I18N] Fix ByteEncoding crashes when using EncoderFallback

Added proper support for EndoderFallback for classes derived from ByteEncoding (i.e. CP1254). This fixes random crashes especially when using SGen garbage collector.
Also added Tests stub for I18N.MidEast.
Changes were done by applying modifications to the code generation tool in tools/ucm2cp.c and running the script tools/cvt.sh. This means that all modifications done to the codepage classes *directly* are lost! Ideally direct changes to codepage classes must be inspected with care, and merged into ucm2cp.c if they are necessary after these changes.
This code has not been tested much.