Handle walk builtin as a loop expression (#86)

The walk builtin generates values and implicitly creates a loop over the values.
Hoist walk calls as loops and handle them. Also handle cases where return value
is bound to an extra parameter.

Closes #83

tests/opa.passing

@@ -76,6 +76,7 @@ partialobjectdoc
 partialsetdoc
 planner-ir
 rand
+reachable
 regexfind
 regexfindallstringsubmatch
 regexisvalid
@@ -108,4 +109,5 @@ units
 urlbuiltins
 uuid
 varreferences
-virtualdocs
+virtualdocs
+walkbuiltin