[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
I think you need FUNC_ATTR_NONNULL_ALL here because you are not checking for arg and tm being NULL anywhere. Same for `f_reltime` and `f_reltimestr` (actually in my opinion every second function needs `FUNC_ATTR_NONNULL_ALL` something like every fourth `FUNC_ATTR_WARN_UNUSED_RESULT`). But these are static ones so such change does not contribute much.