Release the Python GIL when running jsonnet code.
Release the Python GIL when running jsonnet code.

This is a bit of a PoC as it's not perfectly clean, but it does seem to
work fine.

See issue #813

sparkprime/jsonnet,sparkprime/jsonnet,google/jsonnet,sparkprime/jsonnet,google/jsonnet,sparkprime/jsonnet,google/jsonnet,sparkprime/jsonnet,google/jsonnet,google/jsonnet,google/jsonnet,sparkprime/jsonnet