Adding documentation for subtyping

[Eckaos]

In this pull request, I will add the manual documentation for subtyping(Record/Dictionary, Array/Array, Dictionary/Dictionary) and an integration test to chain functions on records.

[yannham]

The beginning is a bit dry. The best way to introduce a notion is to motivate it, with an example. For example, start by saying something like:

While distinct types are usually incompatible, some types might actually be safely converted to some other types. Take the following example:

let extended : {_ : Number} =
 let initial = {foo = 1} in
 std.record.insert initial

And then go on to explain that technically the types don't match, but it's legal to "forget" some information to go from a record type to dictionary type.

And go on to explain why this is accepted while the types don't really match specifically.

[Eckaos]

I added motivation for subtyping. I was wondering do I add constructors(Array, Dictionary) in this pull request too or do I make another pull request to update the manual ? @yannham

[yannham]

I think you can add it to this PR. It feels a bit artificial to split this in several ones, given the small changes all focused on the same file.

[Eckaos]

I don't understand why my array example does not work.

I didn't add to the manual Record/Record and Dyn subtyping for now because it is not implemented.

[Eckaos]

I added a generic example for expanding a function to work around the fact that we don't have arrow subsumption.
Do you think, the documentation miss something else ? @yannham

[yannham]

Suggested change

	dictionary of numbers. In the typing jargon, `{foo : Number}` is said to be a
	a dictionary of numbers. In the typing jargon, `{foo : Number}` is said to be a