Add support for passing char[] as input, allow reuse of given buffer

[cowtowncoder]

Currently it is possible to hand a byte[], which avoids needing to allocate underlying read buffer, as well as one or more read(s). But there is no similar counterpart for char[]. So let's add it.

Doing so could also allow some minor optimization for the case of passing in a String; if profiling shows it is faster to just explicitly get underlying characters, we could do that on JsonFactory side, instead of building a StringReader.