Fix use proc_macro statements for newest nightly/beta

See this for more information:
rust-lang/rust#54418

src/diag.rs

@@ -12,7 +12,7 @@
 //!    `.err()` on spans.
 //!
 
-use proc_macro::{Span, TokenStream};
+use crate::proc_macro::{Span, TokenStream};
 
 
 /// Extension trait that adds a convenience method to `Diagnostic`. This is

src/gen.rs

@@ -1,4 +1,4 @@
-use proc_macro::Span;
+use crate::proc_macro::Span;
 use proc_macro2::TokenStream as TokenStream2;
 use quote::{ToTokens, TokenStreamExt};
 use syn::{

src/lib.rs

@@ -4,10 +4,11 @@
 
 #![cfg_attr(feature = "nightly", feature(proc_macro_diagnostic, proc_macro_span))]
 
+extern crate proc_macro;
 #[macro_use]
 extern crate quote;
 
-use proc_macro::TokenStream;
+use crate::proc_macro::TokenStream;
 use proc_macro2::TokenStream as TokenStream2;
 use quote::ToTokens;
 

src/proxy.rs

@@ -1,6 +1,6 @@
 use std::iter::Peekable;
 
-use proc_macro::{token_stream, TokenStream, TokenTree};
+use crate::proc_macro::{token_stream, TokenStream, TokenTree};
 
 use crate::diag::SpanExt;
 
@@ -141,7 +141,7 @@ fn eat_type(iter: &mut Peekable<token_stream::IntoIter>) -> Result<ProxyType, ()
 
 #[cfg(test)]
 mod test {
-    use proc_macro::TokenStream;
+    use crate::proc_macro::TokenStream;
 
     use super::parse_types;
 

src/spanned.rs

@@ -1,4 +1,4 @@
-use proc_macro::{Span, TokenStream};
+use crate::proc_macro::{Span, TokenStream};
 use proc_macro2::TokenStream as TokenStream2;
 use quote::ToTokens;