Add SimpleIndexer bindings for on-the-fly indexing

[lintool]

This allows "on-the-fly" indexing - so we don't need to first write files to disk and then index.

Very bare bones... no tests, no documentation, etc.

But, works on MS MARCO passage:

python scripts/msmarco-passage/index_msmarco_passage.py \
  --input /System/Volumes/Data/store/collections/msmarco/passage \
  --index indexes/lucene-index-msmarco-passage

On my personal machine (iMac Pro), running time:

Total 8841823 docs indexed in 1284s

This is roughly 21m, which is roughly consistent with figures reported in castorini/anserini#2016 - 18m on the Java end... add a bit of time for Python overhead, so passes sanity check.

PR not ready for review yet.

[ola13]

What format of the data is currently expected in input?

[lintool]

I'll demonstrate in a test case when I get a chance, but the same "standard" JSON format that Pyserini/Anserini takes:

{"id": "0", "contents": "The presence of communication amid scientific minds was equally important to the success of the Manhattan Project as scientific intellect was. The only cloud hanging over the impressive achievement of the atomic researchers and engineers is what their success truly meant; hundreds of thousands of innocent lives obliterated."}

I realize right now that it takes a string representation of JSON, which create a lot of needless string manipulation... but let's save as TODO later and get the pipeline working first.

[lintool]

I added basic documentation and a test case. @crystina-z @ToluClassics this is ready for review now.