Refactor assignment.

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

testdata

@@ -411,6 +411,10 @@ null
 [-1,1,3]
 [0.5, 1.5, 2.5]
 
+.foo += .foo
+{"foo":2}
+{"foo":4}
+
 .[0].a |= {"old":., "new":(.+1)}
 [{"a":1,"b":2}]
 [{"a":{"old":1, "new":2},"b":2}]