…en there is a record/dictionary subsumption)

Co-authored-by: Yann Hamdaoui <yann.hamdaoui@gmail.com>

Co-authored-by: Yann Hamdaoui <yann.hamdaoui@gmail.com>

Co-authored-by: Yann Hamdaoui <yann.hamdaoui@gmail.com>

Co-authored-by: Yann Hamdaoui <yann.hamdaoui@gmail.com>

This commit performs minor cosmetic improvements in the code handling
subtyping, mostly around comments and function interfaces.

The support of subtyping for record types makes some types to be
instantiated earlier than before. Given the way terms are visited, when
writing something like `let f = std.array.map in ..`, `std.array.map`
used to be inferred to be of a polymorphic type `forall a b. ...` and
then only was instantiated (because record access is inferred, while the
bound expression is checked). With the new implementation, it is
instantiated as part of the subsumption rule, meaning that it'll appear
differently on the LSP.

All in all, both version of the data shown by the LSP (before this
change and after) are meaningful, in some sense. The polymorphic type is
still shown when it's part of the metadata anyway, in addition to the
inferred monomorphic type. This also doesn't change which expressions
are accepted or not. It's more of an artifact of when we visit terms,
before or after instantiation and how those visits are intertwined with
`check` and `infer`. It's not easy to revert to the previous state of
affairs, and it's in fact not necessarily a good thing either: in the
example above, the LSP would also show a polymorphic type for `f` - the
one of `map` - while in fact `f` has a monomorphic type, so you couldn't
use it in different contexts (say on an `Array Number` and later on a
`Array String`). The current version is showing the real monomorphic
nature of `f`.