Add javadoc around ByteStream and Environment

[DavyLandman]

I'm trying out metal, and found some parts less documented than others.

undocumented:

• ByteStream (for example, what should be the return value for the read when it reads past the EOF?)
• Environment
• ParseGraph
• Source
• CallBacks

next to it, there is no reference implementation forByteStream.

[jvdb]

Will be added for the upcoming release.

A basic implementation for ByteStream exists in the util package in the test code of the core component. It is exposed in a test-jar.