fix: Handle parsing corner cases (#309)

A block with a single or expression needs to be treated as a comprehension instead of a
set/array with 1 item. e.g.: {1 | 1 }, [2 | foo]

Allow successfully parsing object comprehensions as rule body
x if { 1:2 | 1 }

fixes #306, fixes #307

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

tests/opa.passing

@@ -227,3 +227,4 @@ v1/uuid
 v1/varreferences
 v1/virtualdocs
 v1/walkbuiltin
+v1/withkeyword