add runtime benchmarking to rules

[kzuberi]

Looks like we need to add a command to the various rules to tell the build system to store benchmark stats in some file: benchmark rules

So we could do something like:

rule RULE_A:
    input: whatever
    output: whatever
    benchmark: WORK+"/benchmarks/RULE_A.json"
    shell: whatever

so basically adding that extra 'benchmark:" line to the rule. Unfortunately it looks like we have to do this for all rules we want to measure. I'd recommend trying it on a couple and having a look at the output file and deciding if this will be useful.

@haroldr you might be interested in trying this later.