Concrete syntax for handler types

We should extend the parser (and fix the type pretty-printer) to support handler types. We agreed on the following syntax:
```
handler {E} T_cap in [E_in] T_in => [E_out] T_out
```
Optionally, the binder `{E}` and effects can be omitted. Moreover, it would be nice to support the shortened form
```
handler {E} T_cap => [E_out] T_out
```
when `[E_in] T_in` and `[E_out] T_out` are (pairwise) the same.