[sequences] no memory -> partial memory

[stonier]

• restart the sequence, but allow children to retain their state
• invalidate any potential dangling runners if the sequence halts mid-way

Resolves #329.

(apologies, this was a long time in coming)

[stonier]

There is a followup to this that provides eliminates the inconsistency in default settings for memory between Selector and Sequence → #370.

NB: PR 370 goes further to enforce explicit argument configuration for composites - it should help ease a transition and ultimately make composite code a little more introspectable.