Add apply function for type stable calls of functions

[Tortar]

Fixes #120

[Tortar]

Example

julia> using LightSumTypes

julia> @sumtype X(Bool, Int, Vector{Bool}, Vector{Int})

julia> xs = [X([1,2]), X(true)]
2-element Vector{X}:
 X([1, 2])
 X(true)

julia> sum_(x::X, y::X, z) = apply(sum_, (x, y, z))
sum_ (generic function with 1 method)

julia> sum_(x, y, z) = sum(x) + sum(y) + z
sum_ (generic function with 2 methods)

julia> sum_(xs[1], xs[2], 2.0)
6.0

julia> @code_warntype sum_(xs[1], xs[2], 2.0)
MethodInstance for sum_(::X, ::X, ::Float64)
  from sum_(x::X, y::X, z) @ Main REPL[4]:1
Arguments
  #self#::Core.Const(Main.sum_)
  x::X
  y::X
  z::Float64
Body::Float64
1 ─ %1 = Main.apply::Core.Const(LightSumTypes.apply)
│   %2 = Main.sum_::Core.Const(Main.sum_)
│   %3 = Core.tuple(x, y, z)::Tuple{X, X, Float64}
│   %4 = (%1)(%2, %3)::Float64
└──      return %4