Java front-end: jsr / ret support is missing

[smowton]

Specifically seen in edu/emory/mathcs/backport/java/util/concurrent/ConcurrentHashMap.class, jsr is a simple lightweight call-subroutine opcode (simply pushes a return address) and ret is the corresponding return, performing call/return within a compiled function. They are deprecated, but Java 5 compilers generate them and library JARs built to that standard are evidently still around.

smowton@fc186b4 provides a preliminary implementation.