Please provide a Writer that encodes to String

[joshtriplett]

I'd like to use base64 as a Writer, so that I can use a serde serializer followed by base64, without an intermediate buffer. However, base64::write::EncoderWriter always wraps another Writer. I'd like to get a String out, so that I have a type-level guarantee of valid UTF-8 (which base64 will always produce). Would you consider adding a base64::write::StringWriter that writes to a String, and returns that string from finish()?

[marshallpierce]

That sounds reasonable.

[marshallpierce]

Does #142 address your use case?

[joshtriplett]

@marshallpierce Yes, that looks great! I left one comment there, about avoiding a copy of the data, but otherwise that would work perfectly for my use case.

[marshallpierce]

Released in v0.13.0.