proposal: short => rewrite for function declarations

[Adam D. Ruppe]

On Saturday, 10 October 2020 at 20:43:55 UTC, Steven 
Schveighoffer wrote:
> 1. (v) => v.foo;
> 2. auto x(v) => v.foo;

The latter is almost guaranteed to issue a compile time error 
anyway but it should be simple enough to make it an error to have 
any anonymous parameter in this short syntax for declarations.

Maybe I can code it into the patch, it shouldn't be too 
complicated.