factor with_context guesswork into its own function
spy/objects: break SpyFile, part one

Iterating a SpyFile now doesn't store anything in that SpyFile's .lines,
which is good for two reasons: I've gone off SpyFile's tendency to guess
what you want, and iteration is now a bit faster.

Contrary to 'break SpyFile' this doesn't actually make any tests fail,
which hopefully indicates that this feature didn't matter anyway. :)