- Closes#2188.
This pr introduces a new syntactical statement for defining aliases:
```
syntax alias newName := oldName;
```
where `oldName` can be any name in the expression namespace. Fixity and
module aliases are not supported at the moment.
- The `newName` does not inherit the fixity of `oldName`. We have agreed
that the goal is to inherit the fixity of `oldName` except if `newName`
has a fixity statement, but this will be done in a separate pr as it
requires #2310.
