Issues with multiple instances of the same class in class contexts

The 2018-12-01 itask compiler has several possibly related issues when a class dictionary contains instances of the same class, as in the example below.

• In build_context_fields of checktypes, fields are generated using only the names of the classes, causing duplicate symbols. When not using the optimising linker, this triggers a multiple definition error for the example below (Clean System Files/test.o:(.data.m7+0x8): multiple definition of `e__test__dconvertable_I_Pconvert')
• Overloading is not correctly resolved for some members of the context. This can be seen in the function f below. If it has the type a -> Int | convertable a all is well, with result type String the overloading cannot be resolved (internal overloading of "convert" could not be solved)

Implementation module:

implementation module test

import StdEnv

class convert a b :: a -> b
class convertable a | convert a Int & convert a String

instance convert Int Int where convert i = i
instance convert Int String where convert i = toString i

f :: a -> Int | convertable a
f i = convert i

Start = f 10

Definition module:

definition module test

class convert a b :: a -> b
class convertable a | convert a Int & convert a String

instance convert Int Int
instance convert Int String

f :: a -> Int | convertable a

@johnvg