Add an inverse of u32split instruction

[hackaugusto]

At the time we only a instruction to split a felt into two u32 called u32split, but we don't have the reverse operation to join two u32 to form a felt.

[hackaugusto]

The instruction could be something like:

u32assert2 mul.4294967296 add

edit: actually, now that I think of it, the code above is not always correct, having the value 2**32 - 1 twice in the stack would pass the above check, but the addition would not fit in a felt. more thought needs to be put into the validation