JSStackFrame Evisceration

This would leave sizeof(JSStackFrame) == 6 words.

Sorted in estimated order of benefit / difficulty:

Task	Size (wks)	Assignee	Note
argc/argv	1.5	lw	Requires CallSegment (CallStack) changes, educate decompiler
thisv	.5	lw	Depends on argv/argc
fun, script	.5	lw	Depends on thisv
ncode	3	dvander	Merge it with savedPC in method-jit. Requires building map HW PC --> bytecode (which we sortof already heave
XdisplaySave	1	cdleary	Remove display optimization for great justice!
hookData	.2		Just use js::Invoke and use a local variable
annotation	3	sayrer	Trivial if we can remove callers in nsScriptSecurityManager. Alas, that is not trivial, so this is going to take a while.
callerVersion	.5	cdleary	Mostly the challenge is just understanding the actual use case.
blockChain	1	lw	A bit tricky
rval	1		Tricky
imacpc	1	cdleary	Leave uninitialized in call path and use JSStackFrame::flags to indicate whether there is or is not an imacpc.