We add support for `return` and `raise` statements in the control flow
graph: we simply add an edge to the terminal block, push the statements
to the current block, and proceed.
This implementation will have to be modified somewhat once we add
support for `try` statements - then we will need to check whether to
_defer_ the jump. But for now this will do!
Also in this PR: We fix the `unreachable` diagnostic range so that it
lumps together consecutive unreachable blocks.
This PR contains the scaffolding for a new control flow graph
implementation, along with its application to the `unreachable` rule. At
the moment, the implementation is a maximal over-approximation: no
control flow is modeled and all statements are counted as reachable.
With each additional statement type we support, this approximation will
improve.
So this PR just contains:
- A `ControlFlowGraph` struct and builder
- Support for printing the flow graph as a Mermaid graph
- Snapshot tests for the actual graphs
- (a very bad!) reimplementation of `unreachable` using the new structs
- Snapshot tests for `unreachable`
# Instructions for Viewing Mermaid snapshots
Unfortunately I don't know how to convince GitHub to render the Mermaid
graphs in the snapshots. However, you can view these locally in VSCode
if you install an extension that supports Mermaid graphs in Markdown,
and then add this to your `settings.json`:
```json
"files.associations": {
"*.md.snap": "markdown",
}
```
