Add blob literal support

[senderista]

Fixes #894. This depends on blob literal support in RACO, which has already been merged (uwescience/raco#562).

[coveralls]

Coverage increased (+0.04%) to 26.778% when pulling 3cf932e on blob_literal into d4fdb71 on master.

[coveralls]

Coverage increased (+0.07%) to 26.8% when pulling 77ab6d7 on blob_literal into d4fdb71 on master.

[jingjingwang]

I only have a high-level question: why would we need BitSetExpression? If this change is only for testing purpose then I don't see why we need to transfer a byte array to a BitSet and eventually to a boolean[].

[senderista]

BitsetExpression was just a "cool idea" and seemed like a nice way to test blob types with a flatmap apply. I don't have a specific use case for it, although I hope there will eventually be one :)

[jingjingwang]

Okay, how about calling it something like BlobToBooleanExpression instead of BitSet? A 'BitSet' is only used internally in the Java expression. (I wish there is a way to transfer a ByteBuffer to a boolean[] without using BitSet)

[senderista]

Actually the name was deliberate; the other choices might have been "bitvector" or "bit array". The idea is that the input is interpreted as a bitvector (sequence of booleans) rather than a sequence of bytes. I think a name like BlobToBoolean wouldn't convey that interpretation clearly.