Allows for this form of assert_eq! macros:
```rust
assert_eq!(
left.id, right.id,
"IDs are not equal: {:?} {:?}",
left, right
);
```
Also allows for assert! macros to have the format arguments split across
multiple lines even if the assert condition is not simple:
```rust
assert!(
result >= 42,
"The result must be at least 42: {:?}",
result, result.code, context
);
```
This commit adds a `codemap` module, and moves the `CodemapSpanUtils`
added in #857 to it. This is preparation for adding more `Codemap`
specific utilities.
Refs #434
