Retain computation results with @btime

[giordano]

Save @btime output

This is asked every other week, so probably we should have it directly in BenchmarkTools: julia> @eval BenchmarkTools macro btimed(args...) _, params = prunekwargs(args...) bench, trial, result = gensym(), gensym(), gensym() trialmin, trialallocs = gensym(), gensym() tune_phase = hasevals(params) ? :() : :($BenchmarkTools.tune!($bench)) return esc(quote local $bench = $BenchmarkTools.@benchmarkable $(args...) $…