Require return type annotation for async functions to be of promise type

[cristianoc]

• Add annotation@res.returnType to type annotation on the return type of async functions
• Don't reformat type annotations on the body of async functions
• Add the type constraint on the return type after promoting the body of async functions

[ryyppy]

Thanks for the comparison! This helped greatly.

For reference, this is the way async/await does its type annotations rn:

// async function with inline annotations
let someFn = async (num: int): int => {
  num + 1
} 

// async function in an interface declaration
let someFn: int => promise<int>

Following observations from my side, now having both versions at hand:

• less complexity in the compiler (may not be a justification for the user perspective).
• less characters to type, and especially since we will often write a lot of boxed types promise<result<int>> it would help with readability.
• the body and return type declaration are symmetric in case of inline annotations. This is nice, and TS users I've asked would prefer all the short version.
• The use of promise in an interface type makes sense, since there's no way to use an async keyword neither in a function definition, nor in an external. It also seems easy to document.

[cristianoc]

Closing this as it won't be pursued.