refactor(memory): convert the allocator into an interface

[jsternberg]

This converts the allocator to an interface so we can wrap it in
different locations. It expands on the arrow allocator to add a direct
access to the account method of the underlying resource allocator.

The original plan was to separate these two so the allocator would not
have an Account() method. This proved to be too difficult as the
Account() method is used in some very deep places in the code and
changing all of those locations was just not practical.

Fixes #4472.

Done checklist

• docs/SPEC.md updated
• Test cases written

[onelson]

The diff looks good. Questions for my own edification.

I was a little unsure about when we should be favoring GoAllocator versus ResourceAllocator or even relying on the Allocator interface. Maybe there could be a short explainer in the module-level docstring for memory.

In the diff, you change to use ResourceAllocator some of the time, but not all of the time. Some text about the rationale might help.

[onelson]

Go lang question: seems like this is saying "this interface should match the fields on this other struct" with this line. Is this sort of like serde's flatten attribute?

Also, I was a little confused since this is the memory package, so it seemed ambiguous as to what was going on here. My guess is memory.Allocator here is referring to arrow's memory, but it took me a beat to realize this.

[jsternberg]

Yes, memory references arrow's memory package. What this is saying is "This interface needs to implement this other interface and can be used in places that are expecting that interface." It's not really like flatten but it's a builtin part of the language.

You can read about it in the language spec: https://go.dev/ref/spec#Interface_types.

[jsternberg]

GoAllocator is the name of the same type in arrow that implements a version of the allocator that just uses make(). Since we're adding to the interface, it's just creating a version of that that works with our interface.