lean-rust-parser port of (23 Jun) https://github.com/rust-lang/rust/blob/4429659e4745016bd3f26a4a421843edc7fbc422/compiler/rustc_ast/src/ast.rs (maybe + little of https://github.com/dtolnay/syn/blob/master/src/expr.rs) printer https://github.com/rust-lang/rust/blob/4429659e4745016bd3f26a4a421843edc7fbc422/compiler/rustc_ast_pretty/src/pprust/state/expr.rs