Add CustomTime time mode

[Lysxia]

This option allows users to supply their own time function. (My use case is to get mutator time from the RTS.)

Addresses #60 minus the sub-issue of preventing breaking changes in the future if this option is not general enough.

[Bodigrim]

I like the escape hatch! But given that we are extending TimeMode anyway, shall we add explicit cases for mutator_cpu_ns and/or mutator_elapsed_ns as well?

[Lysxia]

OK. I've added the options as regular values that use CustomTime, because it doesn't seem like there is a use in making them constructors. Or am I missing something? I even added wallTime and cpuTime as lower-cased values for consistency. We could also remove the constructors then, but the breakage seems unnecessary at this point.

[Bodigrim]

Making them constructors would allow to specify them via command-line arguments which IMHO is quite useful.

tasty-bench/src/Test/Tasty/Bench.hs

Lines 916 to 919 in 2a66522

	parseValue v = case v of
	"cpu" -> Just CpuTime
	"wall" -> Just WallTime
	_ -> Nothing

[Lysxia]

Can't we do this

 parseValue v = case v of 
   "cpu" -> Just CpuTime 
   "wall" -> Just WallTime 
   "mutcpu" -> Just mutatorCpuTime
   "mutwall" -> Just mutatorWallTime
   _ -> Nothing

?