Check ObjectType formatting

[jitsedesmet]

I think ProGuardCORE might benefit from a check on the formatting of the Field Descriptors of ObjectType. I've now already forgotten to write the ; twice and since no error is thrown, it's a pain to discover this mistake.

example:

val pattern =
            instructionSequenceBuilder.getstatic("java/lang/System", "out", "Ljava/io/PrintStream")

Is missing a ; in `Ljava/io/PrintStream, yet it is hard to discover this when debugging since no error is thrown.