Unify FormSubmission and AttackFormSubmission

[glv]

These two classes are an embarrassment of duplicated code. The default fuzzing strategy implemented in FormSubmission should be implemented as (say) BasicAttackHandler, and AttackFormSubmission should be renamed to FormSubmission with BasicAttachHandler included by default in the set of attacks.

(As part of this, the distinction between fuzzers and attacks can be eliminated in Crawler; a fuzzer is an attack, and the artificial distinction is confusing. Note that this will be an incompatible change, because the distinction is exposed in test cases. But it's still the right thing to do.)

[glv]

Fixed by 837628b