Switch to in-place indenting

[matklad]

the

https://github.com/rust-analyzer/rust-analyzer/blob/182a2b8e72eee2faaef0ef17330937bd0edaa4b6/crates/syntax/src/ast/edit.rs#L185-L203

api exist to change indent level on syntax nodes. It works by returning a copy of the node with indentation applied.

Long after the introduction of that API, we've switched to mutable API for editing syntax nodes, so now the preference is to modify data in-place. We should introduce new trait for in-place indentation changes, and then update all the usages of the old trait, one by one (no need to do everything in one go).