Make isless, <, >, <=, and >= consistent with == and isequal

[devmotion]

Currently, the inequality operators for Duals are inconsistent with the definitions of == and isequal. In particular, isless violates the first requirement listed in its docstring:

If `isless(x, y)` is defined, then so is `isless(y, x)` and `isequal(x, y)`, and exactly one of those three yields true.

The PR fixes these inconsistencies.

[mcabbott]

To summarise what this does, tagged version is like so:

julia> using ForwardDiff: Dual

julia> pr(x, op, y) = println("x ", op, " y = ", op(x,y));

julia> pr.(Dual(1,1), (>,>=,==,<=,<), 1);  # like pr.(1.0, (>,>=,==,<=,<), 1.0);
x > y = false
x >= y = true
x == y = true
x <= y = true
x < y = false

(@v1.12) pkg> st ForwardDiff
Status `~/.julia/environments/v1.12/Project.toml`
  [f6369f11] ForwardDiff v0.10.36

master has instead this:

julia> pr.(Dual(1,1), (>,>=,==,<=,<), 1);
x > y = false
x >= y = true
x == y = false
x <= y = true
x < y = false

(jl_I2PrM4) pkg> st ForwardDiff
Status `/private/var/folders/yq/4p2zwd614y59gszh7y9ypyhh0000gn/T/jl_I2PrM4/Project.toml`
  [f6369f11] ForwardDiff v0.11.0-DEV `~/.julia/dev/ForwardDiff`

With this PR,

julia> pr.(Dual(1,1), (>,>=,==,<=,<), 1);  # like pr.(1.01, (>,>=,==,<=,<), 1.0);
x > y = true
x >= y = true
x == y = false
x <= y = false
x < y = false

julia> pr.(Dual(1,-1), (>,>=,==,<=,<), 1);  # like pr.(1-0.01, (>,>=,==,<=,<), 1.0);
x > y = false
x >= y = false
x == y = false
x <= y = true
x < y = true

(jl_FcBNXv) pkg> st ForwardDiff
Status `/private/var/folders/yq/4p2zwd614y59gszh7y9ypyhh0000gn/T/jl_FcBNXv/Project.toml`
  [f6369f11] ForwardDiff v0.11.0-DEV `https://github.com/JuliaDiff/ForwardDiff.jl#dw/isless`

A similar change was proposed in #377, and also discussed in #609. Probably this PR closes both of those.