Remove more #include directives from util/util.h.

This time, it was also necessary to remove using declarations,
which meant adding "std::" to everything. I left std::string
alone for now, but added a couple of TODOs for it.

Change-Id: Idc45d6587be3ac3a2be1c9c941b4bad5b7a3fdb3
Reviewed-on: https://code-review.googlesource.com/5443
Reviewed-by: Paul Wankadia <junyer@google.com>