Invalid integer literals are accepted

[kryptan]

Integer literals with suffix starting with e are accepted by proc_macro2 but not by proc_macro or Rust compiler.

    let _: TokenStream = "0einvalidSuffix".parse().unwrap();

This parses with proc_macro2 but panics with proc_macro.

Such literals are also rejected by Rust compiler: playground.

According to Rust reference INTEGER_LITERAL must end with SUFFIX_NO_E.