Return @go.log_timestamp result by parent variable

[mbland]

Currently @go.log_timestamp returns its result by printing it to standard output. While this is the way one would typically return single values from shell functions, it's not ideal in this framework, since practically every other function uses a __go_* variable defined by the parent to return the value. This is to make the framework as fast as possible by creating as few subshells as possible, as those subshells add up quick in this context—especially on Windows, where fork() isn't directly supported, and requires an expensive workaround. (This last note needs to go in the go-script-bash coding/testing guide I need to write per #29.)