proc_macro: Move subspan to be a method on Span in the bridge

This method is still only used for Literal::subspan, however the
implementation only depends on the Span component, so it is simpler and
more efficient for now to pass down only the information that is needed.
In the future, if more information about the Literal is required in the
implementation (e.g. to validate that spans line up as expected with
source text), that extra information can be added back with extra
arguments.

library/proc_macro/src/lib.rs

@@ -1379,12 +1379,7 @@ impl Literal {
     // was 'c' or whether it was '\u{63}'.
     #[unstable(feature = "proc_macro_span", issue = "54725")]
     pub fn subspan<R: RangeBounds<usize>>(&self, range: R) -> Option<Span> {
-        bridge::client::FreeFunctions::literal_subspan(
-            self.0.clone(),
-            range.start_bound().cloned(),
-            range.end_bound().cloned(),
-        )
-        .map(Span)
+        self.0.span.subspan(range.start_bound().cloned(), range.end_bound().cloned()).map(Span)
     }
 
     fn with_symbol_and_suffix<R>(&self, f: impl FnOnce(&str, &str) -> R) -> R {