InputStream Json rendering

Related to #300

Would be a lot easier to supply Json to various things if one could generate an `InputStream` from a `JsonValue`. Right now the only way to do it is with `PipedOutputStream`, which means one either has to sacrifice control over threads to get the writing done asynchronously or do no better than holding the whole Json string in memory.

A pull-based interface for Json serialization, i.e. one that exposes creating an `InputStream`, would work around having to figure out `Flow.Publisher` (the other Java-y way of solving this problem) which is unnecessarily difficult to implement correctly.

The issue does not name files or tests. Start by reviewing the existing JsonValue serialization API and related issue #300, then define the intended pull-based InputStream contract; done requires an agreed API for consuming serialized JSON without requiring a complete in-memory string.

Written by the indexing model from the issue text.