Add decref/incref by reaching into the struct.

This is explicitly warned against in jansson's source,
but NativeCall can't use the json_incref/decref functions,
because they're defined as macros.
Add push/pop, shift/unshift. Rename len to elems.

This brings Jansson arrays more in line with native p6 ones.
add test for array slice
readme/comment about incref