struct vector should use data pointer

[seanyoung]

Some ideas:

• Rather than having the data concatenated to struct vector, make it have a data pointer
• The length should follow the data pointer
• The size is not necessary. We add a function to the heap to tell us the size of the array

This would make struct vector the same as a slice on Solana system calls. The same struct for slice and vector sounds very nice.

Another idea:

• Rather than data pointer + lengh, have a first and last pointer. Then accessing the last entry is cheaper. However, is this ever necessary in Solidity?