Adding capability to auto-bind a type to interfaces it implements

[dabbertorres]

Implements #242.

I think motivation for this was covered well by @bradleypeabody in the linked issue, so I won't go into detail here, but I'll copy over a statement I liked:

The Go language itself resolves interfaces based solely on method signatures, not an explicit implements mechanism - I don't see why wire wouldn't follow the same behavior. wire.Bind can still be used to resolve ambiguous cases.

Summary

Public API

• Addition of wire.AutoBind() and corresponding AutoBinding type.

Internals

• Addition of a new AutoBinding type to the internal/wire package that records a concrete type and position of its token.
• AutoBinding fields added to the ProviderSet, providerSetSrc, and ProviderType structs.
• Check for fully unused AutoBindings.
• processAutoBind() to handle the parsing
• The code to actually implement the auto-binding behavior was added to (*ProviderSet).For().
  This makes it easy to give explicit wire.Bind()s precedence. If a type is not found in the providerMap, and it is an interface, the []AutoBindings list is checked for a type that implements the interface. If one is found, it is added to the providerMap and srcMap to avoid future searches for the same type. If no such type is found, behavior is as before.

Tests

• A simpler InterfaceAutoBinding showing automated binding of Bar -> Fooer.
• A more complex InterfaceExplicitBindingPreferredOverAutoBinding, which is fairly self-descriptive.

I've tried out my fork on a project that was already using wire, and so far it is working rather nicely. It has made the wire.Build()s a bit nicer I think - especially for cases where one concrete type implements 2 or more interfaces.
Coupling is reduced too, since the implementation and the interface don't need to directly know about each other. Usage seems more "Go-ish" from my (obviously a bit biased) point of view.

Any feedback/comments/concerns is appreciated! This was my first time reading through wire's code (though I've used it a lot), so I'll be happy to make any changes necessary if there are concerns around the structure of the implementation.

[zombiezen]

Hey @dabbertorres, thanks for the PR! I'm a bit crunched for time for the next month, so I'm not sure when I will have time to review, but rest assured that this is on my radar.

@vangent or @shantuo, if you have more bandwidth to review, feel free. I'm okay with the direction of this PR (see the discussion on #242), but I haven't looked into any of the specifics.

[eliben]

I don't think we're going to accept this PR for now. Nothing wrong with the PR itself, but wire is in maintenance mode. It does what we need it to do for go-cloud, and works well for other users as well. Being slightly more explicit than necessary at times seems like a reasonable tradeoff for us, valuing simplicity.

I'll keep the PR open for now so interested users can find and apply it if they really need this functionality.