support native C++ modules w/ thiscall

[graydon]

We want to bridge to C++ objects at some point. Avoiding, for the time being,
the misery of vtables: add support for first-class native modules (constructed
over an address value) that thiscall through to name-mangled C++ member
functions.

[graydon]

Shifted to rustc

[brson]

SpiderMonkey is going to have more C++ APIs and less C. This could be a good opportunity to figure out how to do C++ bindings, plus if we can tell them what we can and can't do they can design with us in mind.

I can't recall any discussion of this recently.

[sanxiyn]

See #5853.

[catamorphism]

Nominating for milestone 3, feature-complete (it's the oldest bug and I thought some action should be taken on it :-)

[graydon]

dupe of #5853