Python version exercise 4.4 is not consistent with compile_and_test function in support code

[DarshalShetty]

The first line of exercise 4.4 states:

Implement the allocate_registers pass.

However, the compile_and_test function in utils.py in the python support code doesn't handle a allocate_registers pass.

Also some information about how to add the functions build_interference (exercise 4.3) and uncover_live (exercise 4.2) into the compiler pipeline would be helpful since the Racket version of the book explicitly says that these functions need to be added as new passes.

[jsiek]

Thanks for noticing this! I've updated the exercise descriptions to be explicit. For Python there's just one pass, assign_homes, but it makes use of three functions (uncover_live, etc.).