Use two DWT channels for software tasks instead

[tmplt]

At present (read: when implemented correctly; #43) we rely on a single DWT channel to communicate that a task has entered/exited. If we have

#[trace]
sw_task1() { ... }

#[trace]
sw_task2() { ... }

then we will receive the value 0 on the used DWT comparator when sw_task1 enters/exits and 1 for sw_task2. This is not very stable. Instead, two channels should be used: the first one for tasks that are entered, and the other for tasks that exits. This approach does not require us to record the state of the software tasks because we need only forward the events, just like for hardware tasks.

[tmplt]

Additionally — once implemented, and assuming RTIC Scope is correct — a software task that is entered twice before exited would highlight a reentrant error in RTIC or a hardware issue.