Simplify `run_compiler` control flow.

I find `Compilation::and_then` hard to read. This commit removes it,
simplifying the control flow in `run_compiler`, and reducing the number
of lines of code.

In particular, `list_metadata` and `process_try_link` (renamed `rlink`)
are now only called if the relevant condition is true, rather than that
condition being checked within the function.