Change std imports to core where possible

Useful for seeing what are the gaps toward being compatible with
no-std. Useful for copying chunks of code into a no-std crate.

src/ext.rs

@@ -1,7 +1,5 @@
 use super::ToTokens;
-
-use std::iter;
-
+use core::iter;
 use proc_macro2::{TokenStream, TokenTree};
 
 /// TokenStream extension trait with methods for appending tokens.

src/ident_fragment.rs

@@ -1,6 +1,6 @@
+use core::fmt;
 use proc_macro2::{Ident, Span};
 use std::borrow::Cow;
-use std::fmt;
 
 /// Specialized formatting trait used by `format_ident!`.
 ///

src/runtime.rs

@@ -1,11 +1,11 @@
 use crate::{IdentFragment, ToTokens, TokenStreamExt};
-use std::fmt;
-use std::iter;
-use std::ops::BitOr;
+use core::fmt;
+use core::iter;
+use core::ops::BitOr;
 
+pub use core::option::Option;
 pub use proc_macro2::*;
 pub use std::format;
-pub use std::option::Option;
 
 pub struct HasIterator; // True
 pub struct ThereIsNoIteratorInRepetition; // False
@@ -48,8 +48,8 @@ pub mod ext {
     use super::RepInterp;
     use super::{HasIterator as HasIter, ThereIsNoIteratorInRepetition as DoesNotHaveIter};
     use crate::ToTokens;
+    use core::slice;
     use std::collections::btree_set::{self, BTreeSet};
-    use std::slice;
 
     /// Extension trait providing the `quote_into_iter` method on iterators.
     pub trait RepIteratorExt: Iterator + Sized {

src/to_tokens.rs

@@ -1,11 +1,9 @@
 use super::TokenStreamExt;
-
+use core::iter;
+use proc_macro2::{Group, Ident, Literal, Punct, Span, TokenStream, TokenTree};
 use std::borrow::Cow;
-use std::iter;
 use std::rc::Rc;
 
-use proc_macro2::{Group, Ident, Literal, Punct, Span, TokenStream, TokenTree};
-
 /// Types that can be interpolated inside a `quote!` invocation.
 ///
 /// [`quote!`]: macro.quote.html