Logger is now initialized with its WriteCloser (which can never change
after initialization) and then has its properties modified via method
calls. These method calls return new copies of the Logger, all writing
to the same original WriteCloser still. This allows for doing things
like having a Logger within some entity that has its own KV, for
example.
Also I revamped the merging code a bit, and fixed a precedence
inconsitency in ErrWithKV and CtxWithKV.
