Add memory allocation shortcut and simple variables

[pothos]

The sus binary is a nice playground to run some syscalls, even from a "script" file.
It would be nice to have a way to run syscalls that require a buffer address.
My current workaround would be to take some random heap or stack address or construct an mmap call which is tedious.
Since the CStrings already land on the heap, it would be nice to also get some way to create such a buffer with a given size and be able to refer to it in other syscalls.
One could think of defining a malloc(X) shortcut that constructs a heap array of bytes with the given size.
Since variables aren't parsed yet, it would be nice to have a way of saying x = mysyscall() and in this case x = malloc(123) and then use the variable x in a following syscall write(2,x,123).