Your job for this assignment is to write a Mips assembler that maps a small
subset of Mips assembly into machine code, and then an interpreter for the 
machine code. Please note that the beq instruction has been altered to take
aa offset directly instead of a label. This is done to keep things simple
- otherwise you would need to keep track of labels and their corresponding 
offsets! 

Your job for the assem function found in mips_sim.ml is to map a list of 
Mips assembly instructions into a starting state. A state consists of a 
register file, memory and program counter. Thus, the job of the assem function
is to put the 32-bit encodings of each mips assembly instruction in memory,
start the PC at the beginning of the machine code, and provide an empty
register file. The interp function can then take this initial state and 
produce a final state by "executing" the machine code.