Functions to store nodes

Prior to this fix, functions stored a string representation of their
contents. Switch them to storing a parsed source reference and the
tnode of the contents. This is part of an effort to avoid reparsing
a function's contents every time it executes.

src/parser.cpp

@@ -489,7 +489,7 @@ int parser_t::get_lineno() const {
         // If we are executing a function, we have to add in its offset.
         const wchar_t *function_name = is_function();
         if (function_name != NULL) {
-            lineno += function_get_definition_offset(function_name);
+            lineno += function_get_definition_lineno(function_name);
         }
     }
     return lineno;