Issue Description:

Currently, we require the vjp to be defined in the same generic context. However, this is mathematically wrong and practically limiting.

extension Tensor where Scalar : Numeric {
  @differentiable(vjp: _vjp_mul)
  public static func * (lhs: Scalar, rhs: Tensor) -> Tensor {
    return Tensor(lhs) * rhs
  }
}

In this example, the original function is defined under constraint `Scalar : Numeric`. But it should not be differentiable unless 'Scalar : Differentiable'. For this, we have syntax support for a trailing 'where' clause in '@differentiable':

@differentiable(where Scalar : Differentiable)

But this is currently being ignored by the type checker. We should support it now in order to unblock defining primitives for tensor methods that take or return `Scalar`. With 'where' clauses type-checked, VJP/adjoint should be allowed to be defined in a more constrained generic context.