Create Protobuf.Presence module

[v0idpwn]

This PR is on-top of #398.

Adds a unified presence tracking module. Makes the different encoders use it when deciding whether to skip a field or not.

Adds a public API for checking field presence on Protobuf.Presence.field_presence/2. This is similar to the feature present on the official protobuf libraries, but with a different API.

In Ruby, for example, fields that have explicit presence tracking generate a has_{field_name}? method:

message = MyMessage.new()
message.has_foo? #=> false
message.foo = 0
message.has_foo? # => true

If another field, say bar, doesn't have explicit presence tracking (e.g.: it is a non-optional, non-one_of proto3), it simply doesn't generate the has_bar? method, so you can't "presence-check" these fields.

My proposed API is a bit different, and allows presence-checking :bar, but sometimes giving
a :maybe as a response:

message = %Message{}
Protobuf.Presence.field_presence(message, :foo)
#=> :not_present
Protobuf.Presence.field_presence(%Message{foo: 0}, :foo)
#=> :present

Protobuf.Presence.field_presence(%Message{}, :bar)
#=> :maybe
Protobuf.Presence.field_presence(%Message{bar: 0}, :bar)
#=> :maybe
Protobuf.Presence.field_presence(%Message{bar: 1}, :bar)
#=> :present

Motivation

I think it makes sense to centralize presence checking for a few reasons:

1. Presence rules become even more complex with protobuf editions and I'd like to add support for them soon, so this refactoring seems opportune
2. Existing logic is a bit ad-hoc and duplicated. Seemed like a good opportunity to improve the code I noticed when adding Protobuf.Text.
3. Official protobuf libraries provide APIs for presence checking and I don't see why this one shouldn't

[v0idpwn]

I will merge this to #398, and continue working from there. If you have any feedback, you can give it either here or there :)

[whatyouhide]

I love this but I would expose the function on the Profobuf module and make Protobuf.Presence private, thoughts?

[v0idpwn]

Great idea, will do!