proposal: short => rewrite for function declarations

[Adam D. Ruppe]

On Saturday, 10 October 2020 at 23:22:30 UTC, Paul Backus wrote:
> IMO this is a good idea regardless.

Yeah, I've said before we should deprecate the whole {} thing and 
tell people to use () {} instead in all cases.

But minimally the => {} pattern specifically should be recognized 
and prohibited. The lexer itself could complain about the 
character sequence itself.