RFC: extend linalg and array operations to work on Numbers when it makes sense

As discussed in #1829 and #1843, Julia treats a `Number` as an iterable container (supporting `for`, `ref`, `length`, etc), which makes it much easier to write polymorphic functions that can accept either `Number` or `Array` arguments.  For the same reason, it makes sense to have several functions on arrays also work for `Number`s where it makes sense and the meaning is unambiguous, e.g. `nnz` and `findmin`.

Similarly, as discussed in #763, it is consistent with both mathematical principle and mathematical practice to have many of the linear-algebra functions, from `norm` to `eig`, work on `Number` arguments.   (`inv` already worked.)  Again, this makes it easier to write polymorphic functions that take `Vector`, `Matrix`, or `Number` arguments where it makes sense.