C++: Use getter functions of Token

They are already inlined. Now it's easier to find read-only accesses.

Change-Id: I9aaeca3bc5860e3a20a536a2484925e4334c005f
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
C++: Clarify units of a Token

This will avoid confusion when later more length and indices methods are
added.

In Token:
    length() --> bytes()
    begin() --> bytesBegin()
    end() --> bytesEnd()

Change-Id: I244c69b022e239ee762b4114559e707f93ff344f
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>