Intrinsic pack replaced by pointers in get_params and get_gradients

[jvdp1]

This PR proposes to replace some intrinsics pack by pointers in get_params and get_gradients.
Pro: These changes reduced the times of update on my own tests by ~30%.
Con: these procedures must be impure

[milancurcic]

Thanks, @jvdp1, I'll test.

[milancurcic]

I observe 12% speedup in the MNIST training example with this change. It's a good one. Thanks!

[jvdp1]

@milancurcic As discussed, using my datasets, here are the wall clock times for the steps forward-backward and update:

v0.16.1

• forward-backward: 4.81s
• update: 9.44s

v0.17.0

• forward-backward: 4.80s
• update: 5.61s

So, about a 40% speedup for the step update alone.