Convert most of rust_run_program.cpp to rust (issue #2674).
auto merge of #6140 : Dretch/rust/run-with-rust, r=graydon

Even more of `core::run` could be rust-ified -- I believe that access to the C extern environ can be done with rust now. I did not do this because some special casing is needed for OSX and I don't have a mac I can test with.

I think this will also fix #6096.