parser: implement fn name using keyword

[yuyi98]

This PR implement fn name using keyword.

• Implement fn name using keyword.
• Add test.

struct Foo {}

fn (f Foo) lock() int {
	return 22
}

fn (f Foo) mut() bool {
	return true
}

fn select() bool {
	return true
}

fn lock[T](t T) T {
	return t
}

fn match() bool {
	return true
}

fn main() {
	f := Foo{}
	assert select()
	assert match()

	assert lock[int](22) == 22
	assert lock(22) == 22
	assert lock[string]('hello') == 'hello'
	assert lock('hello') == 'hello'

	assert f.lock() == 22
	assert f.mut()
}

PS D:\Test\v\tt1> v run .  

[spytheman]

Why though?

That is introducing more context sensitivity in the parser.

It will make all tooling harder to make, and the parsing potentially slower.

Is the benefit of being able to fn mut() {} for example, worth it?

[yuyi98]

Sorry, I just don't like the following

	cb.mutex.@lock()
        ...

fn (mut s UdpSocket) @select(test Select, timeout time.Duration) !bool {
	return @select(s.handle, test, timeout)
}

I think it should be more user-friendly.

[spytheman]

I think it should be more user-friendly.

I understand, but I see positives and negatives to both viewpoints, so I can not decide :-| .

@medvednikov what do you think?

[medvednikov]

I agree with both of you.

I hate mutex.@lock and @select.

I also don't like the possible confusion it can introduce.

I'm going to merge it however. Great job!