Implement OpAssign traits with references on the right hand side

[bluss]

AddAssign and the others are only implemented for equal right hand side arguments. AddAssign<u32> for u32 works well for integers, but the by reference Rhs implementation is important too, so that bigints and other non-Copy data types can have some interface in common with the primitive number types.

[nodakai]

By not providing AddAssign<&i32>, you can assume where T: AddAssign<T> applies to lightweight/primitive numbers and where T: AddAssign<&T> applies to bignums

[ottworks]

Creates inconsistencies between x = x / y and x /= y, requiring an unnecessary dereference for iteration: https://is.gd/DIyCYY

[bluss]

@nodakai Then generic code can only handle either of them, but not both.

[alexcrichton]

This was attempted in #41336 but unfortunately it caused too much breakage, so I'm going to close.

[OnlyLys]

Heads up for future readers, this was fixed in Rust 1.22.0.