Fixed reaction conflict and cycle detection

Previously, reaction conflict detection would only detect conflicts
that occurred within one propagation (cycle). Now this detection occurs
across multiple cycles. Added test case for this.
Added new detection for endless reaction loops - each reaction may only
run up to 10000 times per propagation, and will then be failed. Added
test case for this.
Refactored paths, added implicits

Path reactions now use an implicit Txn in apply methods, and have an
alternative "now" method that accepts a shelf and creates a new
transaction.
Added SheetBuilder for transact, and SheetDemo to show use of it.
Removed unused TSwingView.
Added Implicits for transact, currently helps with path reactions by
converting closures to them.