fix: infer return type in implicit return arrow function

[HerrCai0907]

It makes resolveFunctionExpression can return correct function type for expression like (x: i32) => x

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[MaxGraey]

Nice! Please add tests for this feature. Or modify existing tests. For example from:
https://github.com/AssemblyScript/assemblyscript/blob/main/tests/compiler/std/array.ts#L783

let newArr = arr.map<f32>((value: i32) => <f32>value);

to

let newArr = arr.map((value: i32) => <f32>value);
// or
// let newArr = arr.map(value => <f32>value);

[MaxGraey]

Thanks!

[MaxGraey]

Btw this works properly now:

export function test(arr: i32[]): f32[] {
  return arr.map((x: i32) => <f32>x);
}

but that one still required explicit type arg for map:

export function test(arr: i32[]): f32[] {
  return arr.map(x => <f32>x); // ERROR TS1140: Type argument expected.
}

also:

export function test(arr: i32[]): string[] {
  return arr.map((x: i32) => x.toString()); // ERROR TS1140: Type argument expected.
}

Will be great to fix such more usual case as well if possible