Mitigating the attribute proliferation - attribute inference for functions

[Walter Bright via Digitalmars-d]

Attribute inference works when the source to a function must exist (i.e. be 
available to the compiler). Currently, it is done for template functions and 
function literals.

At one point, I had implemented it for auto return functions, because the source 
must exist for them, too, but it got a lot of resistance and was dropped.