Allow both type-constaint only and value, to used.

[oxinabox]

this kinda works, but you need to have it ordered right,
and it always prints a warning.

julia> @stub bar
(::Stub{:bar}) (generic function with 1 method)

julia> @expect bar(::Int)=false
false

julia> @expect bar(1)=true    # should not warn
┌ Warning: Expectation already set
│   name = :bar
│   argvals = (1,)
│   sig = (Int64,)
└ @ Main ?:0
true

julia> bar(1)
true

julia> bar(2)
false

julia> @stub foo
(::Stub{:foo}) (generic function with 1 method)

julia> @expect foo(1)=true
true

julia> @expect foo(::Int)=false  # should not warn
┌ Warning: Expectation already set
│   name = :foo
│   argvals = (ExpectationStubs.DoNotCare{Int64}(),)
│   sig = (Int64,)
└ @ Main ?:0
false

julia> foo(1)  # Wrong!
false

It should always prioritize value over type constraint