add support for closure calls

Compiling languages with closures often requires passing
an extra environment parameter to the called function.

One solution is to use a convention, and reserve, say,
the first argument for that purpose.   However, that
makes binding to C a little less smooth.

Alternatively, QBE now provides a way to remain fully
ABI compatible with C by having a "hidden" environment
argument (marked with the keyword 'env').  Calling a
function expecting an environment from C will make the
contents of the environment undefined, but the normal
arguments will be passed without alteration.  Conversely,
calling a C function like it is a closure by passing
it an environemnt will work smoothly.

all.h

@@ -264,8 +264,12 @@ enum Op {
 	/* function instructions */
 	Opar = NPubOp,
 	Oparc,
+	Opare,
+#define ispar(o) (Opar <= o && o <= Opare)
 	Oarg,
 	Oargc,
+	Oarge,
+#define isarg(o) (Oarg <= o && o <= Oarge)
 	Ocall,
 	Ovacall,