Pass `args` to `run` instead of storing it in a field. This avoids the
need to clone it within `run`.
Also, change `args` from `Vec<String>` to `&[String]`, avoiding the need
for some vecs and clones.
Simplify the `run` macro to avoid sometimes unnecessary dependency
on `TyCtxt`. Instead, users can use the new internal method `tcx()`.
Additionally, extend the macro to accept closures that may capture
variables.
These are non-backward compatible changes, but they only affect
internal APIs which are provided today as helper functions until we
have a stable API to start the compiler.
