Use of string sentinel OMITTED = "<OMITTED>" with identity checks is fragile

[mhliu0001]

While reading the option-handling code, I noticed that strax uses a string literal as a sentinel value:

OMITTED = "<OMITTED>"

and frequently checks it via identity, e.g.

if self.default is not OMITTED:
    ...

The usage of identity checks is fragile. We may want to use a more robust approach, like defining a dedicated class, implementing its deterministic hash, and defining how to unpickle the object. For example:

class _OmittedType:
    __slots__ = ()
    def __repr__(self): return "<OMITTED>"
    def __reduce__(self): return "strax.config._get_omitted", ()

def _get_omitted():
    return OMITTED

OMITTED = _OmittedType()

And in strax.utils.hashablize:

if obj is OMITTED:
    return "<OMITTED>"

Please let me know whether this makes sense. Happy to submit a PR for this.