Allow omitting return type in callback arrow functions

[emillaine]

Fixes #819.

Also makes the linter detect arrow function return types that are now unnecessary, and fixes those cases.

Also I think npm clean should be npm run clean in the readme, so fixed that as well.

[MaxGraey]

Hi, thanks for PR. You should add yourself to NOTICE file

[emillaine]

@MaxGraey Added myself to NOTICE.

[dcodeIO]

LGTM to get things working today, potentially improving the requiresReturnType check in the future. What the compiler does there internally is, if the contextual type has a function signature, it allows omitting types and compiles the function expression according to the given signature instead. Means: A return type is necessary if that's not the case, like if the d.ts uses any, Function or a placeholder like T with type parameters omitted on the call.