unsupported_type `char`

with following defeinitions: 

```erlang
-type digit() :: $0..$9.
-type zero():: $0.

proper_gen:pick(?LET(X, digit(), X)) % works fine

proper_gen:pick(?LET(X, zero(), X)) % fails with `Error: The typeserver encountered an error: {unsupported_type,{char,{16,18},48}}.`
```

which is surprising as both are valid erlang type definitions