Add broadcasting versions of element-wise binary operators

[joelberkeley]

Element-wise operators binary (==), (+) etc. have native support for broadcasting, which may be faster than broadcasting operands first. We don't currently expose this, but could easily do so by adding versions of these operators, say (.+) which broadcast only one operand (two sounds fiddly and unlikely to be used - does numpy/jax even support that?) - here the left one.

Additional operators would prevent user error, and improve clarity of code and type inference. It would be trivial to implement, though tiresome to test.