Proposal: Add `cisd` function for argument in degrees

[jondeuce]

I found myself needing to define a convenience function cisd(theta) = cis(deg2rad(theta)), and given that Base already exports sincosd, cosd, sind, and tand, it seems reasonable to add cisd as well.

A proposed implementation that mirrors the cispi implementation could be:

"""
    cisd(x)

More accurate method for `cis(deg2rad(x))` (especially for large `x`).

See also [`cis`](@ref), [`cispi`](@ref), [`sincosd`](@ref), [`exp`](@ref), [`angle`](@ref).

# Examples
julia> cisd(180)
-1.0 + 0.0im

julia> cisd(45) ≈ 1/sqrt(2) + im/sqrt(2)
true
"""
function cisd end
function cisd(theta::Real)
    s, c = sincosd(theta)
    Complex(c, s)
end

function cisd(z::Complex)
    v = exp(-deg2rad(imag(z)))
    s, c = sincosd(real(z))
    Complex(v * c, v * s)
end

I'm happy to open a PR with some tests if this seems reasonable.

[oscardssmith]

seems reasonable to me, but tagging triage since it is new public api.