Simplify sytax trees API

[matklad]

Looks like I've found a way to significanlty simplify our syntax tree API: instead of using Foo<'a>/FooNode and SyntaxNodeRef<'a>/SyntaxNode we'll be using &'a Foo/TreePtr<Foo> and &'a SyntaxNode/TreePtr<SyntaxNode>, where TreePtr has a deref impl.

This is analogous how one may work with &'a Datum/Arc<Datum>, and indeed TreePtr is Arc underneath.

This is both a good news (should be a massive simplification of API) and a bad news (rewrite everything)

It might also be the case that I am just crazy and sleep deprivated and the idea does not work after all :-)

I'll write a full description tomorrow, here's the PoC: rust-analyzer/rowan@c3f719d

[csmoe]

assign to me!

[matklad]

@csmoe I’d prefer to handle this myself, given the amount of changes here :-)

[csmoe]

@matklad okay :)

[matklad]

Some docs about the new impl: rust-analyzer/rowan#9