Implement yield from C hooks.

Get number of multiple results from C frame.
Add lj_cont_hook: restores multres and dispatch to static ins.
Can use fastcall for lj_dispatch_ins() now.

doc/changes.html

@@ -55,6 +55,7 @@ to see whether newer versions are available.
 <div class="major" style="background: #d0d0d0;">
 <h2 id="snap">Development Snapshot</h2>
 <ul>
+<li>Implement yield from C hooks.</li>
 <li>Add abstract C call handling to IR.</li>
 <li>Improve KNUM fuse vs. load heuristics.</li>
 <li>Drive the GC forward on string allocations in the parser.</li>