Add C API functions (and tests) for WriteBatchWithIndex

[warrenfalk]

I've added functions to the C API to support WriteBatchWithIndex as requested in #1833.

I've also added unit tests to c_test

I've implemented the WriteBatchWithIndex variation of every function available for regular WriteBatch. And added additional functions unique to WriteBatchWithIndex.

For now, the following is omitted:

1. The ability to create WriteBatchWithIndex's custom batch-only iterator as I'm not sure what its purpose is. It should be possible to add later if anyone wants it.
2. The ability to create the batch with a fallback comparator, since it appears to be unnecessary. I believe the column family comparator will be used for this, meaning those using a custom comparator can just use the column family variations.

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[siying]

Thank you for contributing. I'll merge it after our internal CI passes.