term: cache currently selected charset

vt.c::action_print(), which is in the hot path, needs to lookup the
currently selected charset.

This currently requires two loads; first to load the current
charset *index*, and then another to get _which_ charset is currently
mapped to that index.

By caching the current charset, we only need one load.

We need to update the cache when:

* A different charset index is selected (i.e. SO/SI)
* The charset for the current index is changed (e.g. ‘\E(0’, ‘\E(B’
  etc)
* We restore saved charsets

terminal.c

@@ -2335,6 +2335,7 @@ term_restore_cursor(struct terminal *term, const struct cursor *cursor)
 
     term->vt.attrs = term->vt.saved_attrs;
     term->charsets = term->saved_charsets;
+    term->charset = term->charsets.set[term->charsets.selected];
 }
 
 void