Rename Set to Type

[HeinrichApfelmus]

This pull request introduces the name Type instead of the name Set for all types that are transpiled to Haskell. This change has two motivations:

• Be more consistent with nomenclature from Haskell, where the canonical name for types is Type.
• Avoid the name clash with Set from the containers package.

Using Type instead of Set removes a small, yet pervasive stumbling block for people who know Haskell and want to learn Agda2hs. In principle, people can learn the difference, but in this case, I believe that this is learning effort not well-spent.

(In a future change, it may make sense to introduce Prop in addition to Type in order to better distinguish between propositions about Haskell and types in Haskell — while every Haskell type can be transpiled from a Type, the converse is not true. But that discussion is out of scope for this pull request.)

[omelkonian]

LGTM, just a couple of minor omissions.

[HeinrichApfelmus]

Thanks for your review!

I have just noticed that the documentation still mentioned Set instead of Type; I have changed that as well and also added a small note for Agda users that mentions the renaming.

@omelkonian could you take another look? If you're happy, could you also merge the pull request? — I do not have sufficient permissions to merge anything, this pull request is coming from a fork.