Added output of stacktrace on uncaught exception.
Fixed broken support for BURL.open, used by Open.file with URL.
Added a builtin OS.bootURLLoad.
Added builtins IsArity and GetStructuralBehavior.
Reconciled the pickle and boot-pickle formats.
Added support for the remaining types in bootUnpickle().
Fixed: the needed flag was not GRed in VariableBase.
Added a builtin Record.adjoinAtIfHasFeature.

This is partial work of Record.adjoinAt that succeeds if the
original record already has the feature to adjoin. In this case,
a fast copy is done in C++. Otherwise, the function does not
succeed and AdjoinAt should fallback on a slower implementation.

Eventually, this will be replaced by a complete Record.adjoinAt
as a builtin. But we need this partial implementation to be fast
right now so that PEG.oz is not too slow.
BoostBasedVM sets up a default boot loader.