Implement by_name

[hynek]

As a replacement for C.attribute that doesn't work with slots and is
overall bad design.

I’ve also improved the overall exception situation (and its documentation). Comments anyone?

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #86 into master will not change coverage

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[hynek]

This sort-of fixes #82.

[Tinche]

A thought. What about instead of attrs_attrs being a tuple of Attributes, we make it an ordered dictionary of attribute names to attributes?

It's an internal thing, and fields() can be changed to just return list(attrs_attrs.values()) or something.

[Tinche]

Or, since I'm brainstorming, a namedtuple. That might even give us a prettier API than the dict-like by_name.

[Tinche]

If it's a namedtuple, you could do both:

for a in fields(C):
    pass

and

my_attr = fields(C).my_attr

(Again, just brainstorming, it's too early for me :p)

[hynek]

I kind of forgot, that the whole point of fields() is to enable us to do such changes. :D

The irony of using namedtuples within attrs is left to the reader.

[Tinche]

The thought of using an attrs slot class here did cross my mind, but I was afraid of attrs-ception.

However, namedtuples can't have fields beginning with underscore, and we obviously support attributes with names beginning with underscore. This limitation is probably so you can't clash with a method name, like _asdict, _make or _replace. We are of course better than namedtuple and don't make our users deal with this crap.

I like the API a lot, so let me play around with it, implement it another way.

[hynek]

OK the new API is