More library functions (#51)

- units.parse, units.parse_bytes
- json.is_valid, json.marshal, json.unmarshal
- yaml.is_valid, yaml.marshal, yaml.unmarshal
- object.subset
- set_diff

* Also print number of errors due to each missing function
* Also lock down fully passing OPA suites

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

scripts/pre-push

@@ -14,4 +14,7 @@ if [ -f Cargo.toml ]; then
    if [[ "$OSTYPE" == "linux-gnu"* ]]; then
        scripts/coverage
    fi
+
+   # Ensure that OPA conformance tests don't regress.
+   cargo test --test opa -- $(tr '\n' ' ' < tests/opa.passing)
 fi