document well-typedness of object subtypes

[gregwebs]

Please link to the docs - they are sorely needed!

[taku0]

The subtyping of objects seems very restricted.

The following code is typed:

let f x = x.y.z

f {x: 1, y: {z:1, zz: 2}}

while the following is not:

let f x = x.y.z

let g h = h {x: 1, y: {z:1, zz: 2}}

g f

The following is also typed.

let f x = x.y.z

let g h = h {y: {z:1}}

g f

[taku0]

Typed X(.

let f x = x.y.z

let g h = h {}

g f

[puffnfresh]

@taku0 that's probably because I haven't implemented structural types using type constraints. Lazy/silly/short-sighted of me.

Fixing that is on my TODO list :)

[puffnfresh]

@gregwebs absolutely awesome job. Thanks!