generalize internal representation of generic types

[hishamhm]

All types that have type variables are now represented as a GenericType record, which holds a non-generic Type and an array of type arguments.

This change is because originally we only cared about generic records and generic functions, but once we have the local type MyType<T> = ... syntax, other types can be generic as well (in particular, unions).

Instead of replicating generic support logic in the implementation of each type, we factor it out into a type-level term which encapsulates the application of type variables, which is something more like second-order lambda calculus.

(See https://en.wikipedia.org/wiki/System_F and the ensuing rabbit hole.)

Fixes #880.
Fixes #787.

[github-actions]

Teal Playground URL: https://890--teal-playground-preview.netlify.app

[hishamhm]

I just saw two issues already when running the LuaRocks codebase through it:

Error 1) modules that return a generic type

./luarocks/search.tl:16:55: missing type arguments in generic<K> table.SortFunction<K> | Ordering<K>

https://github.com/luarocks/luarocks/blob/master/src/luarocks/search.tl#L16

Error 2) and a stricter resolution of type arguments (which I need to look closer, but might be a legitimate error that it has now caught)

./luarocks/cmd/write_rockspec.tl:415:81: argument 3: got Ordering<string>, expected table.SortFunction<number | string> | Ordering<number | string>

https://github.com/luarocks/luarocks/blob/master/src/luarocks/search.tl#L16

[hishamhm]

Error 1 there was being misreported (it was being reported at the wrong location). It was a legitimate error, a missing <K>. Fixed here.

Error 2 is a bit trickier because it is a type check that we would like to accept (because we know that Ordering<string> is a valid Ordering<string | number>, but only because we know how the Ordering object works), but we can't, because the compiler has no way to tell that values of the type variable's type are only used for reading and not for writing... typical covariance/contravariance limitation. Currently, type variables are invariant, for simplicity. The easy fix is to change the field to Ordering<string | number> in the LuaRocks codebase.

[hishamhm]

All right, I decided to do the brave thing and merge this already.

The fixes contained here are important enough so that it's clear that this should be the baseline moving forward. If this has introduced any further regressions, then let's fix it in the main branch.