Updated som.sh to use truffle.jar

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Simplified Universe.main, and change it to exit with non-zero code on error

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

This allows to write minimal scripts that do not load the standard libraries.

Invocation looks like this:
`$java som.vm.Minimal -cp Smalltalk:TestSuite/BasicInterpreterTests $ClassName $classMethod`

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Change SClass and SObject constructors to take class in order to avoid invalidation in setClass()

Also adapted the initialization of fields to use @ExplodeLoop and be a little more side-effect-free to help the partial evaluator.

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Add @ExplodeLoop to array initialization as well, but no changes to the implementation

Didn't change the code here, not sure whether the change in SObject's field nil'ing was entirely necessary.

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