Implement std::mem::memcopy procedure

[Fumuran]

This PR implements new memcopy procedure for the std::mem module.

This procedure copies the provided number of Felts from the read pointer to the write pointer. It uses memcopy_words and memcopy_felts procedures internally in order to optimize the copy.

The stack transition looks like so:

Inputs: [n, read_ptr, write_ptr]
Outputs: []

Closes: #2359.

[Al-Kindi-0]

Looks good!
Left some comments and questions

[Al-Kindi-0]

Should we add some proptests?

[Fumuran]

I tried to do so, but the main problem is that we can't assert the correctness of the value copied to some new location: in case of the proptest we can't come up with some formula which will represent the value here. But I'm new to the proptest dependency, so maybe I'm missing something.

[bobbinth]

Let's use the new syntax here (i.e., pub proc). And we should merge the latest next into this to make sure all tests pass against it (i.e., export should no longe be allowed).

Also, I wonder if we can simplify this procedure to not have conditional logic. For example, we could try to compute 3 tuples: (ptr1, n1), (ptr2, n2), (ptr3, n3) and then call memcopy_elements, memcopy_words, and memcopy_elements on these unconditionally.

The idea here is that if we start with a word-aligned address, the first tuple would be (read_ptr, 0) and the second tuple would be (read_ptr, n2) etc.

If it is more efficient, the tuples could be (start_ptr, end_ptr) and in that case, for word-aligned addresses, the first tuple would be just (read_ptr, read_ptr).

The main question here is if we can implement computing these tuples efficiently. We will probably need to have a helper procedure for computing "next multiple of 4" - but I think that could be done relatively efficiently.