add analysis-bench to benchmark incremental analysis

Can be used like this:

```
$ cargo run --release -p ra_cli -- \
  analysis-bench ../chalk/ \
  --complete ../chalk/chalk-engine/src/logic.rs:94:0

loading: 225.970093ms

from scratch:   8.492373325s
no change:      445.265µs
trivial change: 95.631242ms
```

Or like this:

```
$ cargo run --release -p ra_cli -- \
  analysis-bench ../chalk/ \
  --highlight ../chalk/chalk-engine/src/logic.rs

loading: 209.873484ms

from scratch:   9.504916942s
no change:      7.731119ms
trivial change: 124.984039ms
```

"from scratch" includes initial analysis of the relevant bits of the
project

"no change" just asks the same question for the second time. It
measures overhead on assembling the answer outside of salsa.

"trivial change" doesn't do an actual salsa change, it just advances
the revision. This test how fast is salsa at validating things.

crates/ra_project_model/src/lib.rs

@@ -37,6 +37,7 @@ pub enum ProjectWorkspace {
 /// `ProjectRoot` describes a workspace root folder.
 /// Which may be an external dependency, or a member of
 /// the current workspace.
+#[derive(Clone)]
 pub struct ProjectRoot {
     /// Path to the root folder
     path: PathBuf,