Add magic slot to strings.

Could add it to all variables but we only need it on strings for now
(to stash compiled JSONPath expressions) and putting it inside the
string means it benefits from the copy-as-reference behaviour of
strings. It's also available to arrays if necessary - by virtue of them
being string based.
Array creation sugar.
Hash creation sugar.
Merge branch 'jsonpath' of github.com:AndyA/jsondata into jsonpath