eligible for deferred parsing (e.g., not arrow functions, not
functions-in-block). This is experimental behavior, off by default. Define
an 'on' mode in which all eligible functions are redeferred on GC, as well
as a 'stress' mode in which all candidates are redeferred on each stack
probe. This change is built on a previous PR that refactors the
FunctionBody hierarchy to make it easier to toggle between deferred and
fully-compiled states.

…k that Rajat did a while back. The heuristic uses 5 constants: an initial delay, which is the number of GC's we perform before we start checking for redeferral candidates; the number of GC's to wait between redeferral checks while we're in the startup phase; the number of GC's that must have passed since a function was executed before we redefer it in the startup phase; and the same wait and inactivity constants for the main phase, to which we transition after the startup phase ends. Todo: emit jitted code to reset a function's inactivity count when the function is executed, and/or avoid redeferring a jitted function if we think it may be executed again.

…FunctionBody/ParseableFunctionInfo's in the case where we re-compile a function that encloses already-compiled functions. This is necessary to avoid having multiple FunctionInfo structures around for a single FunctionBody, not all of which get updated when things change. It also reduces the cost of re-compiling. 2) Change the FunctionBody's nested function array to hold FunctionInfo's instead of FunctionProxy's. This fixes a class of functional issues involving the need to update the nested array as nested functions are un-deferred and re-deferred. The FunctionInfo pointers don't change and thus don't need updating. 3) Eliminate the m_referenceInParentFunction member of FunctionProxy, which was there to enable the nested array updates that we no longer need. 4) Re-use ScopeInfo in the case where a parent of the current function has been re-deferred.

…ns without generating new code. We make use of the per-entry-point calls count for jitted entry points as well as the interpreted count for the FunctionBody.