Update assistants to use streaming API

[jlowin]

Updates assistants to use the streaming API. Nearly no user-facing impact, but much better UX especially because we can stream outputs live to the terminal:

import marvin
from marvin.beta.assistants import CodeInterpreter

def add(a, b):
    return a + b

# a = marvin.beta.assistants.Assistant(tools=[CodeInterpreter])
a = marvin.beta.assistants.Assistant(tools=[add, CodeInterpreter])

a.say('add one and two using the add tool, then use the interpreter to subtract a random number from it')