The previous massert isn't that useful compared to testify, but it's
nice to have some assertion helpers in the repo which don't require a
sub-dependency.
This change required refactoring nearly every package in this project,
but it does a lot to simplify mctx and make other code using it easier
to think about.
Other code, such as mlog and mcfg, had to be slightly modified for this
change to work as well.
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.
