Convert upcalls to core Rust functions

[brson]

The main motivation is that the new failure code is written in Rust so calling it directly makes more sense than indirecting through native runtime code.

This also lets us to have fewer stack-switching mechanisms as upcalls are dealt with differently than other foreign functions.

[brson]

Probably upcall_call_on_c_stack/call_on_rust_stack needs to remain a native function, same for upcall_new_stack/del_stack (which are not called by generated code but my __morestack).

[brson]

Actually, upcall_call_on_x_stack should be fine as a rust functions. it should just be the upcalls called by __morestack that need to be special.

[elliottslaughter]

This has been (partially) completed for fail in de82a9b and malloc/free in d257382. The current implementation is a very thin wrapper around the C runtime, but in the future it should be possible to move more functionality over to rust.

[graydon]

Agree, properly tagged and milestoned as of 2013-06-05.

[emberian]

@brson is this still relevant?

[brson]

@cmr Pretty much obsolete.