Replace parse_[sth]_expr with parse_expr_[sth] function names

This resolves an inconsistency in naming style for functions
on the parser, between functions parsing specific kinds of items
and those for expressions, favoring the parse_item_[sth] style
used by functions for items. There are multiple advantages
of that style:

* functions of both categories are collected in the same place
  in the rustdoc output.
* it helps with autocompletion, as you can narrow down your
  search for a function to those about expressions.
* it mirrors rust's path syntax where less specific things
  come first, then it gets more specific, i.e.
  std::collections::hash_map::Entry

The disadvantage is that it doesn't "read like a sentence"
any more, but I think the advantages weigh more greatly.

This change was mostly application of this command:

sed -i -E 's/(fn |\.)parse_([[:alnum:]_]+)_expr/\1parse_expr_\2/' compiler/rustc_parse/src/parser/*.rs

Plus very minor fixes outside of rustc_parse, and an invocation
of x fmt.

compiler/rustc_parse/src/parser/ty.rs

@@ -433,7 +433,7 @@ impl<'a> Parser<'a> {
         };
 
         let ty = if self.eat(&token::Semi) {
-            let mut length = self.parse_anon_const_expr()?;
+            let mut length = self.parse_expr_anon_const()?;
             if let Err(e) = self.expect(&token::CloseDelim(Delimiter::Bracket)) {
                 // Try to recover from `X<Y, ...>` when `X::<Y, ...>` works
                 self.check_mistyped_turbofish_with_multiple_type_params(e, &mut length.value)?;
@@ -494,7 +494,7 @@ impl<'a> Parser<'a> {
     // To avoid ambiguity, the type is surrounded by parentheses.
     fn parse_typeof_ty(&mut self) -> PResult<'a, TyKind> {
         self.expect(&token::OpenDelim(Delimiter::Parenthesis))?;
-        let expr = self.parse_anon_const_expr()?;
+        let expr = self.parse_expr_anon_const()?;
         self.expect(&token::CloseDelim(Delimiter::Parenthesis))?;
         Ok(TyKind::Typeof(expr))
     }