func unixtimeNs() uint64 {
	sec, nsec := walltime()
	return uint64(sec*1_000_000_000 + int64(nsec))
}
Standard library's `time.Now()` will invoke two syscalls in Linux. One is `CLOCK_REALTIME` and
another is `CLOCK_MONOTONIC`. In our case, we'd like to separate these two calls to measure
time for performance purpose.
`unixtimeNs()` is identical to Linux's `clock_gettime(CLOCK_REALTIME, &ts)`
package minitrace
import (
	_ "unsafe"
)
func nanotime() int64
func walltime() (sec int64, nsec int32)
func monotimeNs() uint64 {
	return uint64(nanotime())
}