[RFC] Constant PMCs in PBC

[chromatic]

A good optimization step is enabling the use of compile-time constant PMCs in 
bytecode.  Consider this code generated by NQP-rx:

	.namespace []
	.sub "fib"  :subid("11_1259711141.42547") :outer("10_1259711141.42547")
	    .param pmc param_16
	.annotate "line", 3
	    new $P15, 'ExceptionHandler'
	    set_addr $P15, control_14
	    $P15."handle_types"(58)
	    push_eh $P15
	    ...
	.end

The ExceptionHandler in $P15 is (effectively) constant and won't change 
throughout the invocation of the program.  Recreating it (and calling a 
*method* on it to set the single type of exception it will handle) is busy 
work all but the first time.  (This code comes from the NQP-rx fib.pir 
benchmark; we can get an easy 45% performance improvement from avoiding re-
creating this EH on every sub invocation.)

We should be able to create a single PMC once, at compile time, and refer to 
it throughout the rest of the PIR.  To accomplish this, we need:

	* PIR syntax to declare a constant PMC and initialize it at compile time
	* PIR syntax to refer to such a constant PMC
	* tests for both
	* (possibly) a way to refer to local symbols in other subs, such as the label 
used with the set_addr op

Arguably, we could instead use a way to associate an EH with a sub such that 
entering the sub always activates that EH, but then we have to consider access 
to the sub's local symbols within the EH, and that's trickier.

-- c