Merge #3710

3710: Inlay hints for method chaining pattern r=matklad a=M-J-Hooper

This PR adds inlay hints on method call chains:
![image](https://user-images.githubusercontent.com/13765376/77472008-8dc2a880-6e13-11ea-9c18-2c2e2b809799.png)

It is not only explicit `MethodCall`s where this can be helpful. The heuristic used here is that whenever any expression is followed by a new line and then a dot, it resembles a call chain and type information can be  #useful.

Changes:
- A new `InlayKind` for chaining.
- New option for disabling this type of hints.
- Tree traversal rules for identifying the chaining hints.
- VSCode decorators in the extension layer (and associated types).

Notes:
- IntelliJ has additional rules and configuration on this topic. Eg. minimum length of chain to start displaying hints and only displaying distinct types in the chain.
- I am checking for chaining on every `ast::Expr` in the tree; Are there performance concerns there?

This is my first contribution (to RA and to Rust in general) so would appreciate any feedback.
The only issue I can find the references this feature is #2741.

Co-authored-by: Matt Hooper <[email protected]>

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