Improvements and simplification in corpus' handling randomisation code

[jvoisin]

• Remove useless casts to bool
• Use something faster than random.randint: this cuts in half the time spent
  in the random.py file, without raising anything else
• use random.choice where possible, instead of mylist[_rand(len(mylist))]
• Add some comments explaining the if…if…else structure, to prevent
  future refactoring from transforming them into an if…elif…else

On a local fuzzer, with 10000 iterations, this decrease the amount of time
spent in corpus' mutate function from ~0.2 seconds to ~0.1 seconds, for a total
run time of ~9 seconds, which seems to not be a lot, but since 8.60s is spent
in connection.py's poll function waiting for the fuzzer, it's a worthy
optimization.