Add sugar for declaration punning

This is analogous to record (or definition or binding) punning where one
can write e.g.

    x = 1;
    y = 2;
    xy = {x; y};  ;; same as {x = x; y = y}

Now that notation also exists for declarations.

This is motivated by the upcoming intersection like signature combination
operator `&`.  Consider a `MAP` signature:

    type MAP = {
      type t _;
      Key: ORD;
      ;; ...
    };

With the declaration punning notation and intersection like combination, one
could write

    Map (Key: ORD) :> MAP & {Key} = ;; ...

to refine the `MAP` signature.

README.md

@@ -172,6 +172,7 @@ D ::= ...
  X A1 ... An = E ~> X : A1 -> ... -> An -> (= E)
  type X A1 ... An ~> X : A1 => ... => An => type
  type X A1 ... An = T ~> X : A1 => ... => An => (= type T)
+ X ~> X : (= X)
 
 (expressions)
 E ::= ...

parser.mly

@@ -183,6 +183,8 @@ atdec :
  | head typparamlist EQUAL exp
  { VarD($1, funT($2, EqT($4)@@ati 4, Pure@@ati 3)@@span[ati 2; ati 4])
  @@at() }
+ | name
+ { VarD($1, funT([], EqT(VarE($1)@@ati 1)@@ati 1, Pure@@ati 1)@@ati 1)@@at() }
  | TYPE head typparamlist EQUAL typ
  { VarD($2, funT($3, EqT(TypE($5)@@ati 5)@@ati 5, Pure@@ati 4)@@at())
  @@at() }

regression.1ml

@@ -2,6 +2,10 @@ record_inference x = x.works_a_little;
 
 ;;
 
+type DEC_PUNNING = {int; list};
+
+;;
+
 Equivalence: {
  type t a b;