[vpm] optimized codegen avoids option-boxing

introducing two mutable variables per pattern match: matchRes and keepGoing
keepGoing denotes whether the result was Some or None,
and matchRes holds the Some's contents or the right zero for the match's type

Race(() => fastMatch(list), () => virtMatch_no_option(list))(100000).converge() is a virtual tie on my machine after this
see https://gist.github.com/1400910

conveniently also works around SI-5245

don't assign to Unit-typed var's, in fact, make matchRes a val when its only prospect in life is to be unit-valued
propagate eventual type for matchRes thru codegen
so that we can have more robust checks for unit&nothing, when assignment makes no sense

also, added a hack to caseResult to avoid boxed units in
if(keepGoing) { matchRes = ... } else zero
after erasure, we get
if(keepGoing) { matchRes = ...; BoxedUNIT } else zero

genicode broke because i was sharing trees:
[scalacfork] error: java.lang.AssertionError: assertion failed: type error: can't convert from UNIT to REF(class Object) in unit ScalaSig.scala at source-/Users/adriaan/git/scala-dev/src/scalap/scala/tools/scalap/scalax/rules/scalasig/ScalaSig.scala,line-26,offset=868
fixed by duplicating -- so be it (for now -- make this more fine-grained, more efficient)

dodging inliner issues with one/zero (it won't inline, so also directly inline those methods)

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