Bump benchmark iteration count

Author: dtolnay

benches/bench-libproc-macro/README.md

@@ -4,7 +4,7 @@ Example output:
 $ cargo check --release
 
    Compiling bench-libproc-macro v0.0.0
-STRING: 15 millis
-TOKENSTREAM: 97 millis
-    Finished release [optimized] target(s) in 1.06s
+STRING: 37 millis
+TOKENSTREAM: 276 millis
+    Finished release [optimized] target(s) in 1.16s
 ```

benches/bench-libproc-macro/lib.rs

@@ -4,7 +4,7 @@ use proc_macro::{Ident, Punct, Spacing, Span, TokenStream, TokenTree};
 use std::iter::once;
 use std::time::Instant;
 
-const N: u32 = 7000;
+const N: u32 = 20000;
 
 #[proc_macro]
 pub fn bench(_input: TokenStream) -> TokenStream {