Subtyping: better errors

[NoamDev]

fixes #219

This PR introduces better error messages for subtyping failures.

Examples:

Try par check on the following example to reproduce yourself:

module Main

import @core/Int
import @core/String


type Point = either { .a Int, .b String }

def T: [Point] either {} = [x] x

def T2: [[!,<a,b,c>!, !]!][!,<a,b>!, !]! = [x] x

def T3: [[type a: box]!][type b]! = [x]x

def T4: [(type a)!](type b: box)! = [x]x

def T5: [[<a: box>!]!][<b>!]! = [x]x

type Functor<s> = either {.a choice {.b => s}}

def T6: [iterative either {.a choice {.b => self}}]recursive either {.a choice {.b => self}} = [x] x

type Inner<s> =
	choice {
		.none => recursive either {
			.get s,
			.result self,
		}
	}


type ReadP =
	iterative@get
	Inner<self@get>


dec ForceCast :
	[
		Inner<ReadP>
	]
	ReadP
def ForceCast = [p] p


dec Bind :
	[ReadP]
	ReadP
def Bind =
	[p]
	chan res {
		res.begin@get
		p.none.begin
		p.case {
			.get p => {
				res.case {
					.none => {
						res.get
						res.loop@get
					},
				}
			},
			.result p => {
				let foo = chan res {
					p.loop
				}
				let bar = ForceCast(foo)
				res <> bar
			},
		}
	}

def T9: either{} = chan res {
    let x: recursive ! = !
    x.begin
    let x: recursive ! = !
    {loop} <> x
} 

[faiface]

Looks good to me now! Thank you so much.