Move TokenExpander to base_db and rename it

It's only used to break the dependency to proc_macro_api

crates/tt/src/lib.rs

@@ -1,7 +1,7 @@
 //! `tt` crate defines a `TokenTree` data structure: this is the interface (both
 //! input and output) of macros. It closely mirrors `proc_macro` crate's
 //! `TokenTree`.
-use std::{fmt, panic::RefUnwindSafe};
+use std::fmt;
 
 use stdx::impl_from;
 
@@ -247,8 +247,3 @@ impl fmt::Display for ExpansionError {
         }
     }
 }
-
-pub trait TokenExpander: fmt::Debug + Send + Sync + RefUnwindSafe {
-    fn expand(&self, subtree: &Subtree, attrs: Option<&Subtree>)
-        -> Result<Subtree, ExpansionError>;
-}