Standardize "x = x op y" to "x op= y"

Here's one of a series of proposed small simplification/optimizations across the code base. This can be rejected.

The change is to standardize expressions like:

x[i] = x[i] + b;

to:

x[i] += b;

... for any operation that has an 'op=' version. The resulting byte code is very marginally faster since the target is evaluated once; this might matter in a tight loop manipulating a 2D array cell.

There's a minor argument that it is simpler code. Since both styles appear in the code now, this would also represent a tiny standardization.

The counter-argument is that "x += foo" might risk being misread more readily as "x = foo"