[RFC] viml: impl profiling on top of uv_hrtime()

Should be better than gettimeofday() since libuv uses higher resolution
clocks on most UNIX platforms. Libuv also tries to use monotonic clocks,
kernel bugs notwithstanding.

Necessary for Windows, which doesn't have gettimeofday().

Relevant issue: #831

References for `STATIC_ASSERT`:
- http://www.pixelbeat.org/programming/gcc/static_assert.html
- http://comments.gmane.org/gmane.comp.lib.gnulib.bugs/25870
- https://github.com/gagern/gnulib/blob/master/lib/verify.h
- https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61407

**UPDATE**: Since we now have a platform abstract `uint64_t` type (`proftime_T`) instead of `LARGE_INTEGER` and `struct timeval`, I'm thinking of passing by value and perhaps even eliminating some of the one-line functions.

Ping: @justinmk @ZyX-I @equalsraf
It will be 32-bit on windows indeed. But I wouldn't change the prototype of the function just yet (a bit more invasive since it's called with `long` globals and whatnot). How about this:

```c
    *tm = uv_hrtime() + ((proftime_T) msec * 1000000ULL);
```

That should (I think) make it not overflow unless for extremely high values.