Bug fixes to ParallelMatrixDebt prototype.
Checkpointing state.

Checkpointing state in order to to investigate assert fail after
thrice-loading matrix-debt-1.js into js debug shell.
Bug fix for assert-fail: misnamed getter.
checkpoint semi plausible sequential implementation.  No token reuse yet; that is next item on place.
Remove debug instrumentation via mode parameter.
Switching to model where an unpaid debt causes an exception to be thrown.  With a test even.
Code cleanup.

Use func.length (i.e. arity) to decide whether to use the debt-token
protocol.  (The other option is to let caller allocate arrays/matrices
and copy their contents into our target matrix.)

After factoring the above case into its own path, it was much easier
to identify the error checks and put them on their own path as well.