Convert rust_run_program.cpp to Rust

[graydon]

At this point it's really plausible to put almost all this logic in rust, with the exception of the extern global access.

[emberian]

@graydon I can't find rust_run_program anywhere in the tree, I assume this is obsolete?

[Dretch]

@cmr rust_run_program.cpp still exists and I believe at least some of it could be converted to rust (environ access). So I think this bug is not quite obsolete.

[emberian]

Oh, it's a filename! Righty. Editing the issue, as there is no longer a chdir there.

[emberian]

Also a note: the extern global should be implementable in Rust via static mut, I believe.

[graydon]

accepted for production-ready milestone

[thestinger]

Sadly, this is still around.