blocks: implements objects.Block

[zchee]

Implements objects.Block.
The Block interface impl might be ugly, pls advice how to implements it 🙏

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TODO:

• implements each Blocks []interface field

[lestrrat]

• This is a judgement call, but I want structure definitions to live in objects/interface.go (think of it as a .h file in C), so I'd like to see stuff in blocks.go like type XXX struct { } to be moved to that file.
• I don't know exactly how block elements work, but once we publish Block.Elements as []interface{} we will be stuck with that type declaration onwards. I haven't analyzed it yet myself, but I think there should be at least an attempt that trying to define what Go type can go into Block.Elements

[zchee]

@lestrrat Thanks for advice. will fix and implements more structs.

[zchee]

@lestrrat I have almost implemented the Slack block feature. but uses interface on some types.
What do you think about this?

[lestrrat]

One thing that might be good to have: an actual consumer code of your data objects. For example, how do you actually create/consume Block elements? This will tell you if it's a good idea to use interfaces and/or if you have the adequate methods to properly use them.