compile-mode integration: add link to code references.

[mookid]

This allows to jump to code references such as lines 82 and 132 in the
following snippet error message.

error[E0061]: this function takes 1 parameter but 2 parameters were supplied
  --> file6.rs:132:34
    |
82  | fn duration_ms_since(time: &Option<SystemTime>) -> u128 {
    | ------------------------------------------------------- defined here
...
132 |             self.total_time_ms = duration_ms_since(&self.program_start, 2);
    |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[rust-highfive]

r? @brotzeit

(rust_highfive has picked a reviewer for you, use r? to override)