Eliminate use of unrefined types to simplify API appearance

[tel]

Right now the Api type is ugly as all hell

data Api nat symbol star where ...

What is that? nat symbol star? Ugly! Confusing!

We can eliminate (probably?) the uses of nat and symbol if we never actually use unrefined types (see here) though I'm not yet sure how. This will reduce the type noise to just Api star which is easier to explain and less noisy. This is a big improvement!

[tel]

These instances are a bit broken by this change:

instance HeaderEncode 'AccessControlAllowHeaders (Set (HeaderType Text)) where
  headerEncode _ = displaySetOpt . Set.map headerName

instance HeaderEncode 'AccessControlAllowHeaders [HeaderType Text] where
  headerEncode = uniqueSet

[tel]

I could probably replace those with something like Set SomeHeader for SomeHeader = exists (h :: HeaderType Symbol) . Sing h. That's a possible way forward!