[lgfirqfs] Functional debugger watchpoint

A debugger watchpoint will stop when a variable (a location in memory) changes value.  However, we might not be interested in a specific variable, or we might not know what the specific variable is.  Instead, we might be interested in when a pure function of the program state changes value.

Implement such a watchpoint efficiently, without having to reevaluate the function on every clock cycle.  The trick that I hope will work is that the user-specified function is not a black box; the debugger and instrumenting compiler can examine its internals.

Yet another instance of this problem.