This implements the following operations:

* Integer, to push a native int
* AsFixnum, to construct a RubyFixnum from an int
* ARRAY_LENGTH runtime helper to retrieve array's length as an int
* BIntInstr for branch operations against two ints

To support these, the following logic was added to manage int
local variables on the JVM stack:

* TemporaryIntVariable: similar to the prototype unboxed variables
  but stored boxed in the Object[] temp array in the interpreter.
  The JIT knows to use int when dealing with this type of var.
* int-based temps used by the pattern logic are switched to
  TemporaryIntVariable.

A few minor utility functions are added to SkinnyMethodAdapter,
including the oddly missing icmp_ge.