Moved all classes that are not really AST nodes to som.interpreter package

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Moved specialized message nodes into separate files

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Introduced specialized message nodes for #ifTrue:, #ifFalse:, and #ifTrue:ifFalse:

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Whitespace

Signed-off-by: Stefan Marr <git@stefan-marr.de>