[RFC] Add init_pmc() to ExceptionHandler

[chromatic]

While we need to consider ways to make EHs less expensive, a cheap and easy 
gain comes from adding the init_pmc() VTABLE entry in the attached patch.  For 
an EH which handles only a single type of exception, we can rewrite code from:

	.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

... to:

	.sub "fib"  :subid("11_1259711141.42547") :outer("10_1259711141.42547")
	    .param pmc param_16
	.annotate "line", 3
	    $P1 = box 58
	    new $P15, 'ExceptionHandler', $P1
	    set_addr $P15, control_14
	    push_eh $P15
		...
	.end

Note that this replaces a method call with Integer boxing.  The resulting code 
(PIR generated by a Fibonacci benchmark written with NQP-rx) runs 22.166% 
faster.

It might be useful to extend the initializer to take a PMC array as well, when 
the EH can handle multiple types of exceptions, but most of the uses I've seen 
in NQP-rx so far handle only one type of exception.

-- c
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