proc_macro: move some implementation details to a rustc module.

src/libsyntax/parse/lexer/mod.rs

@@ -1775,12 +1775,6 @@ fn ident_continue(c: Option<char>) -> bool {
     (c > '\x7f' && c.is_xid_continue())
 }
 
-// The string is a valid identifier or a lifetime identifier.
-pub fn is_valid_ident(s: &str) -> bool {
-    let mut chars = s.chars();
-    ident_start(chars.next()) && chars.all(|ch| ident_continue(Some(ch)))
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;