[Dev] Improve parsing

Improve parsing to raise errors for wrong call arguments for state / control (~~possible? actually must work if `t0` is defined at eval and equal to `0`...~~)

```julia
ocp = @def begin
t ∈ [0, 1], time
x = (q, v) ∈ R², state
u ∈ R, control
x(t0) == [1, -2] # instead of x(0)
x(tf) == [0, 0] # instead of x(1)
∂(q)(t) == v(t)
∂(v)(t) == u(t)
0.5∫( u(t)^2 ) → min
end
```