std.benchmarking and getting rid of TickDuration

[Jonathan M Davis via Digitalmars-d]

I just created a PR aimed at replacing the few pieces of Phobos 
that use TickDuration in their API, and it involves creating the 
std.benchmark module to hold the updated benchmarking functions, 
since they can't be overloads of the existing ones in 
std.datetime given that return types are involved. So, the new 
functions need to go in a different module. Putting them in 
std.benchmark made perfect sense given that that's where we were 
looking at putting them before when Andrei's std.benchmark was 
reviewed (and ultimately dropped). Normally, adding a module to 
Phobos would involve an extended review in the newsgroup, but in 
this case, it's just updated versions of existing functions moved 
to a new module. So, I don't think that a review of that level is 
required, but I figured that I'd bring it up in the newsgroup 
just in case.

https://github.com/D-Programming-Language/phobos/pull/3695
- Jonathan M Davis