Replace Instruction.getForOpcode() with a constant lookup table

Every time we call Scene.drawCurrentContent(), which happens
repeatedly during the love.draw() routine, we call the getForOpcode()
function to get the LNVL.Instruction object for the current opcode we
are executing.  The mapping between opcodes and instructions never
changes during run-time.  So we can replace the getForOpcode()
function with a constant lookup table.  In theory this should improve
performance but the difference is realistically negligible.  However,
it is still a better approach to take.
Verify every opcode has a matching instruction when in debug mode

Since the map of opcodes-to-instructions is now a table instead of a
function we can easily perform this test in debug mode.  This will
stop is from adding a new opcode and forgetting to provide a
corresponding LNVL.Instruction object for the engine to use.