This commit adds stack-structured timing code, such that bits of SPINS
can be instrumented with

>  timing_push("short name")
>  [... work goes here]
>  timing_pop()

This is recursive, such that already-timed portions of the code can call
the timing code to further instrument subroutines or discrete code
segments, and the resulting structure follows the -dynamic- call stack.
This does have the odd result that portions of the code that are re-used
(namely spectral derivatives) can show up as several 'leaf' nodes in the
graph.

To print out the report to standard error, call

>  timing_stack_report()

The exact format of the report should be considered tentative.