Improve the API for adding tyvars somehow

[andy-morris]

This is awful:

do t' <- newName "t"; let t = varT t
   extendTerm "TypedTerm" [t'] [t|Typed $t|] \_ ->
     defaultExtTerm {
       typeVar   = Just [[t|Type $t|]],
       typeAbs   = Just [[t|Type $t|]],
       typeApp   = Just [[t|Type $t|]],
       typeConst = Just [[t|Type $t|]]
     }

There must be a way to do better.

[andy-morris]

A half-baked idea:

• have a length indexed vector of names
• apply the vars to the tag internally (pass just [t|Typed|] instead of [t|Typed $t|])
• use the vector's length to know how many lambdas to add to the ExtTerm. Something like

extendTerm "TypedTerm" ("t" :> VNil) [t|Typed|] \_v t ->
  defaultExtTerm {
    typeVar   = Just [[t|Type $t|]],
    typeAbs   = Just [[t|Type $t|]],
    typeApp   = Just [[t|Type $t|]],
    typeConst = Just [[t|Type $t|]]
  }