proposal: short => rewrite for function declarations

[Adam D. Ruppe]

After a brief chat yesterday, I slapped this together:

https://github.com/dlang/dmd/pull/11833

In short, in a function declaration, it rewrites `=> ...;` into 
`{ return ...; }`

One benefit is shorter property accessors:

     private int _x = 34;
     @property x() => _x;
     @property x(int v) => _x = v;

But it also works basically anywhere

     bool isNull() => this is null;
     auto identity(T)(T a) => a;
     @property y() in(true) => _x; // contracts still work too

So it just extends the existing lambda shorthand to full 
declarations too.

See more in the PR description and the test case there.