std: Remove extra type params on iter adaptors

Now that associated types are fully implemented the iterator adaptors only need
type parameters which are associated with actual storage. All other type
parameters can either be derived from these (e.g. they are an associated type)
or can be bare on the `impl` block itself.

This is a breaking change due to the removal of type parameters on these
iterator adaptors, but code can fairly easily migrate by just deleting the
relevant type parameters for each adaptor. Other behavior should not be
affected.

Closes #21839
[breaking-change]

src/libsyntax/parse/mod.rs

@@ -436,7 +436,7 @@ pub fn str_lit(lit: &str) -> String {
     let error = |&: i| format!("lexer should have rejected {} at {}", lit, i);
 
     /// Eat everything up to a non-whitespace
-    fn eat<'a>(it: &mut iter::Peekable<(usize, char), str::CharIndices<'a>>) {
+    fn eat<'a>(it: &mut iter::Peekable<str::CharIndices<'a>>) {
         loop {
             match it.peek().map(|x| x.1) {
                 Some(' ') | Some('\n') | Some('\r') | Some('\t') => {
@@ -605,7 +605,7 @@ pub fn binary_lit(lit: &str) -> Rc<Vec<u8>> {
     let error = |&: i| format!("lexer should have rejected {} at {}", lit, i);
 
     /// Eat everything up to a non-whitespace
-    fn eat<'a, I: Iterator<Item=(usize, u8)>>(it: &mut iter::Peekable<(usize, u8), I>) {
+    fn eat<'a, I: Iterator<Item=(usize, u8)>>(it: &mut iter::Peekable<I>) {
         loop {
             match it.peek().map(|x| x.1) {
                 Some(b' ') | Some(b'\n') | Some(b'\r') | Some(b'\t') => {