[LucaCappelletti94]

Block typedefs with a C bool, 31 of them including nw_connection_receive_completion_t and DispatchDataApplier, cannot be constructed, because bool is neither EncodeArgument nor EncodeReturn while msg_send! reserves it for BOOL.

This adds EncodeBlockArgument and EncodeBlockReturn, which extend those traits with C bool and pointers to it, and bounds block2 on them. Blocks use the C ABI, so msg_send! and defined methods are unaffected. DispatchData::to_vec drops its u8 to bool transmute and keeps only a lifetime cast, since the generated applier is 'static despite DISPATCH_NOESCAPE.

Closes #752.