Compiler: access instantiations of NamedTuple and other generics uniformly

[HertzDevil]

Fixes #10380.

#10380 (comment) now works. The following will now correctly fail to compile (with the default prelude):

module Foo
end

struct NamedTuple
  include Foo
end

class Bar
  include Foo
end

def foo(x : Bar)
end

x = uninitialized Foo
foo(x)

Error: no overload matches 'foo' with type Foo

Overloads are:
 - foo(x : Bar)
Couldn't find overloads for these types:
 - foo(x : NamedTuple())
 - foo(x : NamedTuple(file: String))
 - foo(x : NamedTuple(time: Time, location: Time::Location))

This replaces Crystal::GenericType#generic_types with #instantiated_types, so that Crystal::NamedTupleType can redefine it. Note that the keys of Crystal::GenericType#@generic_types and Crystal::NamedTupleType#@instantiations are never accessed outside those compiler types' instantiation methods repsectively (they are only used for caching instantiations with the same type arguments).

[asterite]

This is great! ❤️