Deduplicate fundep environment entries

[jbouwman]

Prevent addition of duplicate fundep entries by moving the no-match case inside the update block.

(A change to fundep environment type signature was proposed and rolled back.)

[eliaslfox]

I don't think this is correct, and I'm confused as to why the tests are passing.

The fundep environment was previously:

ImmutableMap<class, ImmutableMap<fundep-idx, ImmutableList<fundep-entry>>>

This pr changes it to:

ImmutableMap<class, List<fundep-entry>>

Knowing which relations are associated w/ which fundeps is be required. See the following example:

(in-package #:coalton-user)

(coalton-toplevel
  (define-class (C :a :b :c (:a -> :b) (:b -> :c)))
  
  (define-instance (C Integer String Double-Float)))

(in-package #:coalton-impl/typechecker/environment)

(lookup-fundep-environment coalton-impl/entry::*global-environment* 'coalton-user::C)

#S(IMMUTABLE-LISTMAP
   :DATA #{|
            (0
             #[
               #S(FUNDEP-ENTRY :FROM (COALTON:INTEGER) :TO (COALTON:STRING))
               #S(FUNDEP-ENTRY :FROM (COALTON:INTEGER) :TO (COALTON:STRING)) ])
            (1
             #[
               #S(FUNDEP-ENTRY
                  :FROM (COALTON:STRING)
                  :TO (COALTON:DOUBLE-FLOAT))
               #S(FUNDEP-ENTRY
                  :FROM (COALTON:STRING)
                  :TO (COALTON:DOUBLE-FLOAT)) ]) |}/#[ ])

Note how Integer -> String is attached to fundep #1 and String -> Double-Float is attached to fundep #2. I'm not sure about the duplicate entries, that looks like a bug lol.

[jbouwman]

I've reverted changes to fundep environment: this changeset only prevents duplicate entries.