Refactor assignment.

New version is much more elegant and probably slower. Uses
library functions implemented in jq rather than dedicated opcodes.

compile.h

@@ -34,7 +34,6 @@ block gen_subexp(block a);
 block gen_both(block a, block b);
 block gen_collect(block expr);
 block gen_fold(const char* varname, block init, block body);
-block gen_assign(block expr);
 block gen_definedor(block a, block b);
 block gen_condbranch(block iftrue, block iffalse);
 block gen_and(block a, block b);