Fix possible loss of data warnings in MSVC

[pwnall]

Warning texts:
benchmark.h(558): warning C4244: 'argument': conversion from 'int64_t' to 'double', possible loss of data
benchmark.h(564): warning C4244: 'return': conversion from 'const double' to 'int64_t', possible loss of data
benchmark.h(587): warning C4244: 'argument': conversion from 'int64_t' to 'double', possible loss of data
benchmark.h(593): warning C4244: 'return': conversion from 'const double' to 'int64_t', possible loss of data

Thank you very much for maintaining google benchmark!

[coveralls]

Coverage increased (+0.006%) to 89.053% when pulling 73ca0c6 on pwnall:msvc-warn into edc77a3 on google:master.

[AppVeyorBot]

✅ Build benchmark 1502 completed (commit 725183bc2d by @pwnall)

[dmah42]

Thank you!

[pwnall]

Thank you very much for the quick review!

[LebedevRI]

Maybe in 2.0 we could change the signatures of those functions to avoid casts.