Add source string to Span

This is the path to the location that this span is pointing to. This is
usually going to be a file path, but it can really be anything you want.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/compile.rs

@@ -64,9 +64,9 @@ impl<'s> Compile<'s> {
                 // TODO - is self.compile the right thing to do here?
                 let compiled = Rc::new(self.compile(stmts.clone()));
                 let locals = self.scope_stack.pop_scope().unwrap();
-                let quote = self
-                    .quote_table
-                    .insert(expr.span(), locals, stmts.clone(), compiled);
+                let quote =
+                    self.quote_table
+                        .insert(expr.span().clone(), locals, stmts.clone(), compiled);
                 Inst::PushValue(Value::Quote(quote)).into()
             }
         }