rt: get rid of rust_fn and replace with fn_env_pair plus a little cleanup.
rt: take into account alignment for debug_opaque. Closes #2667
rt: Fix alignment in debug_opaque
auto merge of #4915 : luqmana/rust/rt-cleanup, r=pcwalton

Closes #1190.

Also, got rid of `rust_ptr_eq` since it isn't used any as well the `RUST_REFCOUNTED` macros.

Fixes #2667: alignment & wrong structure.