Allow subtracting two @benchmarkables to create a new @benchmarkable

[NHDaly]

I'm not sure if this is even desirable, and if it is, i'm not sure i've done it the best way, but this is something we want to do:

Here's the context for this:

I'm trying to define a BenchmarkGroup for https://juliaci.github.io/PkgBenchmark.jl. But in my existing benchmark file, I'm subtracting the results of two @benchmark runs (one where I do my operation, and one where I don't) in order to get the exact time for just my operation. Is there any way I can reproduce that type of logic with the @benchmarkable structure I need for the BenchmarkGroup?

Here's my existing logic that I'm trying to convert to be a BenchmarkGroup:
https://github.com/JuliaMath/FixedPointDecimals.jl/blob/3e7da851ea9caa0e267c21e0bb067ae32ee9ad77/bench/decimal-representation-comparisons.jl#L96-L99

Ideally, i'd love to just subtract two @benchmarkables:

        bbase = @benchmarkable $fbase()
        bbench = @benchmarkable $fbench()
        SUITE["bench"] = bbench - bbase

but that's not a thing, it seems 😞

is there anything like that? Subtracting two benchmarkables to get a new benchmarkable that returns the diff of the two?

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So this PR adds the ability to do that. I'd love to talk through it with you and figure out the best way to do something like what we're trying to do here! :)