Unify `@function` and `@contractor`

[dpsanders]

Remove @function and make @contractor do that job instead, e.g.

const C = @contractor f(x) = 4x

will automatically define the const function f, such that f(X) just applies the forward function for standard function evaluation, but

f(constraint, X)

does the forward-backward contractor.

[dpsanders]

Probably better to just do

@contractor C(x) = 4x

i.e. define the const name C as the current @function does.

[dpsanders]

No longer relevant