Introduce named arguments
See e.g., https://en.wikipedia.org/wiki/Named_parameter
Suggested syntax:
[parameter name] = [argument]
Suggest that an instance must either use all named or all unnamed arguments, i.e., an instance may not use both.
A missing named argument is taken to be none
.
The motivation for named arguments is to make instances more readable, which seems especially relevant for annotation instances (#27 (closed)).