Stream out

[SymbolixAU]

When parsing to large strings fails, need a method to stream to a connection

Just ran into this on to_ndjson in a production pipe:
Error in rcpp_to_ndjson(x, unbox, digits, numeric_dates, factors_as_string, : R character strings are limited to 2^31-1 bytes
This was an extremely large write and (remarkably) the first issue I've had in ~6Tb worth of json writes since I installed this branch, over lots of different data types/sizes/nesting structures/etc. So far it's all just working.

Originally posted by @sheffe in #29 (comment)