Instead just use `pprust::path_to_string(..)` where needed.

This has two benefits:

a) The AST definition is now independent of printing it.
   (Therefore we get closer to extracting a data-crate.)

b) Debugging should be easier as program flow is clearer.

…=GuillaumeGomez

Add long error explanation for E0697

Part of rust-lang#61137

r? @GuillaumeGomez

…entril

Print lifetimes with backticks

Fixes rust-lang#65287

r? @varkor

syntax: consolidate function parsing in item.rs

Extracted from rust-lang#65324.

r? @estebank

Remove implicit dependencies on syntax::pprust

Part of rust-lang#65324.

The main goal here is to facilitate the eventual move of pprust out from libsyntax and because an AST definition typically should not depend on its pretty printer.

r? @estebank

…ochenkov

refactor session::config::build_session_options_and_crate_config

I'm not exactly sure what happened in rust-lang#65361 (comment) so in an effort to bisect the error I'm extracting out the first commit so it can hopefully land (I suspect the error is in the 2nd-3rd commits.)

r? @petrochenkov

(I'll r=you when the PR builder is happy)

Move `Nonterminal::to_tokenstream` to parser & don't rely directly on parser in lowering

Split out from rust-lang#65324.

r? @petrochenkov

Add some tests for fixed ICEs

Fixes rust-lang#44153 (from 1.23.0)
Fixes rust-lang#47486 (from 1.36.0)
Fixes rust-lang#48010 (from 1.38.0)
Fixes rust-lang#48027 (from nightly)
Fixes rust-lang#48638 (from nightly)