Add propagate function and opcode

This allows better stacktraces when manually intercepting
signals to clean up resources. Also allows functionality
from Common Lisp's unwind-protect, such as calling cleanup code
while unwindinding the stack, restarting on certain signals, and
just in general having more control over signal and signal propagation.

Also fix a bug encountered while implementing with-resource in the
compiler. Desturcturing arguments that were not the last argument
would often result in bad code generation, as slots used to destructure
the earlier arguments would invalidate the later parameters. This is
fixed by allocating all named parameters before doing any destructuring.

src/core/cfuns.c

@@ -92,6 +92,9 @@ static JanetSlot opreduce(
 
 /* Function optimizers */
 
+static JanetSlot do_propagate(JanetFopts opts, JanetSlot *args) {
+    return opreduce(opts, args, JOP_PROPAGATE, janet_wrap_nil());
+}
 static JanetSlot do_error(JanetFopts opts, JanetSlot *args) {
     janetc_emit_s(opts.compiler, JOP_ERROR, args[0], 0);
     return janetc_cslot(janet_wrap_nil());
@@ -297,7 +300,8 @@ static const JanetFunOptimizer optimizers[] = {
     {NULL, do_gte},
     {NULL, do_lte},
     {NULL, do_eq},
-    {NULL, do_neq}
+    {NULL, do_neq},
+    {fixarity2, do_propagate}
 };
 
 const JanetFunOptimizer *janetc_funopt(uint32_t flags) {