Allow function negation -f to work like !f

[mcabbott]

We have !f === (!)∘f to negate any boolean function. Was there ever discussion of making -f === (-)∘f too? Both are small conveniences to avoid defining x -> op(x). This one would allow for example:

julia> A = reshape(1:16,4,4);

julia> eigvals(A; sortby=-real)  # does not accept rev=true
4-element Vector{ComplexF64}:
     36.209372712298546 + 0.0im
 -4.146487327145629e-17 + 1.9341169324764503e-16im
 -4.146487327145629e-17 - 1.9341169324764503e-16im
     -2.209372712298546 + 0.0im

julia> sum(-log, A; dims=1)  # -sum allocates twice
1×4 Matrix{Float64}:
 -3.17805  -7.42655  -9.38261  -10.6846

This will not change the fact that -f.(xs) means -(f.(xs)), when usually fused @. -f(xs) would be better. (Unless you write (-f).(xs) by hand.)

Needs tests obviously, but RFC?

[jariji]

I'm not necessarily opposed to this but I'll raise some downsides.

(1) Defining functionality for functions that doesn't work on general callables means we can't use it in functions that take generic callable argument.

(2) Related, -a is already syntax for AbstractArray while !a isn't, so a callable AbstractArray might have confusing behavior if the -a is different under function application versus indexing. I don't know of any examples like that but there might be some.

(3) I don't like special case syntax: I think it would be better if instead of adding !f and -f to avoid the extra characters in (!)∘f, we could have instead !∘f and -∘f -- without the parentheses -- that would just work and be almost as convenient as the special cases.

[nsajko]

This method seems fit for an independent PR. I think it'd be merged easier than the feature addition.