Omitting effects means "please infer"

[b-studios]

This is a counter proposal to #609.

def show(n: Int): String = <>

should mean

"please infer the effect"

instead of being syntax sugar for

def show(n: Int): String / {} = <>

This is more consistent to omitting types and we can still show the inferred effects in VSCode.

There might be backwards compatibility problems with overload resolution, but we'll see this when implemented.

[b-studios]

When working on this I realized that function definitions are the only place where this is the case. All other usages of Effects need to have the notion of "omitting means pure" since we cannot infer it (for instance in operation declarations).

[jiribenes]

Given the message above, I consider this issue to be incompatible with the current design.
In case you disagree, feel free to reopen.