Add autouse memory

[jlowin]

Add autouse memory to avoid tool calls if possible.

import marvin

marvin.run('remember the user name is XYZ', memories=[marvin.Memory('user')])

# this is a new thread, with no history
result = marvin.run('What was the user name?', memories=[marvin.Memory('user', auto_use=True)])

# prints XYZ, no tool calls were made
print(result)