* [ breaking ] remove parsing of dangling binders
It used to be the case that
```
ID : Type -> Type
ID a = a
test : ID (a : Type) -> a -> a
test = \ a, x => x
```
and
```
head : List $ a -> Maybe a
head [] = Nothing
head (x :: _) = Just x
```
were accepted but these are now rejected because:
* `ID (a : Type) -> a -> a` is parsed as `(ID (a : Type)) -> a -> a`
* `List $ a -> Maybe a` is parsed as `List (a -> Maybe a)`
Similarly if you want to use a lambda / rewrite / let expression as
part of the last argument of an application, the use of `$` or parens
is now mandatory.
This should hopefully allow us to make progress on #1703
A few proofs have been rewritten, a few unnecessary cases cut, and
lots of unnecessary "explicit implicits" have been cut. Probably these
implicits were required when the code was initially written, and
inference has improved since then.
