Check valid punctuation character in Punct::new

[dtolnay]

Closes #470.

Valid punctuation characters generated by:

use proc_macro::{Punct, Spacing, TokenStream};
use std::panic;

#[proc_macro]
pub fn punct_chars(_input: TokenStream) -> TokenStream {
    for ch in '\0'..=char::MAX {
        if let Ok(_) = panic::catch_unwind(|| {
            let _ = Punct::new(ch, Spacing::Alone);
        }) {
            eprintln!("{:?}", ch);
        }
    }
    TokenStream::new()
}