Allow references to be used as scalar operands

[zackangelo]

trafficstars

Fixes #1200

Relaxes the type constraint on StaticOperand so that it may be used for references.

This example now works:

let arr1 = array!([1, 2, 3]);
let four: i32 = 4;
let arr2 = arr1 + &four; //previously errored on lack of an Add<> impl

[zackangelo]

The CI failure appears to be unrelated to the code change:

docker: Error response from daemon: Duplicate mount point: /home/runner/work/ndarray/ndarray.

[bluss]

Removing 'static is a breaking change. 'static implies that one can use Any and ad hoc type based "specialization", which is why we have snuck in 'static in a few places. We would like to continue to have 'static in all places where we'd like to insert explicit SIMD or BLAS etc operations.

[bluss]

closing for now because keeping 'static is preferred