Make constants printable

Since we print them in one of the new error messages

guppylang/tys/const.py

@@ -39,6 +39,11 @@ def unsolved_vars(self) -> set[ExistentialVar]:
         """The existential type variables contained in this constant."""
         return set()
 
+    def __str__(self) -> str:
+        from guppylang.tys.printing import TypePrinter
+
+        return TypePrinter().visit(self.cast())
+
     def visit(self, visitor: Visitor) -> None:
         """Accepts a visitor on this constant."""
         visitor.visit(self)

guppylang/tys/printing.py

@@ -2,7 +2,7 @@
 
 from guppylang.error import InternalGuppyError
 from guppylang.tys.arg import ConstArg, TypeArg
-from guppylang.tys.const import ConstValue
+from guppylang.tys.const import Const, ConstValue
 from guppylang.tys.param import ConstParam, TypeParam
 from guppylang.tys.ty import (
     FunctionType,
@@ -53,7 +53,7 @@ def _fresh_name(self, display_name: str) -> str:
         self.counter[display_name] += 1
         return indexed
 
-    def visit(self, ty: Type) -> str:
+    def visit(self, ty: Type | Const) -> str:
         return self._visit(ty, False)
 
     @singledispatchmethod