Introduce NonterminalKind

It encapsulate the (part of) the interface between the parser and
macro by example (macro_rules) parser.

The second bit is somewhat more general `parse_ast_fragment`, which is
the reason why we keep some `parse_xxx` functions as public.

src/librustc_parse/parser/expr.rs

@@ -1450,7 +1450,7 @@ impl<'a> Parser<'a> {
 
     /// Matches `'-' lit | lit` (cf. `ast_validation::AstValidator::check_expr_within_pat`).
     /// Keep this in sync with `Token::can_begin_literal_maybe_minus`.
-    pub fn parse_literal_maybe_minus(&mut self) -> PResult<'a, P<Expr>> {
+    pub(super) fn parse_literal_maybe_minus(&mut self) -> PResult<'a, P<Expr>> {
         maybe_whole_expr!(self);
 
         let lo = self.token.span;