Refactor builtin Sig into TypeRepr

We used to have two indications of a builtins type:

 -  `Sig i o`, which just provided the arity
 -  `BuiltinType i`, which is a closure that checks the type

However, since `Sig` just indicates the arity, it doesn't give us quite enough
information to render the capabilities doc as described in #224.

In this refactor, I extended `Sig` to `TypeRepr` and made it hold more
information: an actual deep embedding of the types.  I also moved this new type
into `BuiltinType` (which is now a record).  This allows convenient construction
of _both_ `TypeRepr` and the checker closure using `🡒` and `out`.

fregot.cabal

@@ -44,6 +44,7 @@ Library
     Fregot.Eval.Number
     Fregot.Eval.TempObject
     Fregot.Eval.Value
+    Fregot.Eval.Value.Conversion
     Fregot.Find
     Fregot.Interpreter
     Fregot.Interpreter.Bundle

lib/Fregot/Builtins.hs

@@ -20,8 +20,6 @@ module Fregot.Builtins
     ( ToVal (..)
     , FromVal (..)
 
-    , Sig (..)
-
     , Args (..)
     , toArgs
 

lib/Fregot/Builtins/Base64.hs

@@ -33,11 +33,11 @@ builtins = HMS.fromList
     ]
 
 encode :: Applicative m => (B.ByteString -> B.ByteString) -> Builtin m
-encode f = Builtin (In Out)
+encode f = Builtin
     (Ty.string 🡒 Ty.out Ty.string) $ pure $
     \(Cons text Nil) -> pure . T.decodeUtf8 . f $! T.encodeUtf8 text
 
 decode :: Applicative m => (B.ByteString -> B.ByteString) -> Builtin m
-decode f = Builtin (In Out)
+decode f = Builtin
     (Ty.string 🡒 Ty.out Ty.string) $ pure $
     \(Cons t Nil) -> pure . T.decodeUtf8 . f $! T.encodeUtf8 t