V: implement auto op overload for +=, etc when +, etc overloaded

[SleepyRoy]

What do you want?
I propose += op should be automatically overloaded by compiler when + has been overloaded. The behavior of A += B should be automatically defined as A = A + B.
And the same should be applied to other arithmetic ops (-=, *=, /=, %=).

Why do you want this?

1. Compatible with other rules in V. V places many restrictions on op overloading. Now V requires in op overloading both arguments must have the same type. Alex also said in #4538 that the returned type should also be the same as arguments. This makes this proposal possible - A + B always return A's type (also B's type).

1. Useful. Overloading on += etc is commonly used in areas such as scientific computing.

2. Saving time. This is obvious - use += directly.

3. Safe and natural. A += B should do A = A + B. Imagine a piece of code defining behavior of += not like this, I believe other programmers will complain about it.

[medvednikov]

Yes I think this will be supported.