This change adds support for `tellp`, `tellg` and seeking.
**Additional notes:**
+ `tellg` calls `rdbuf->pubseekoff(0, cur, out)` [lib.ostream.seeks] so need to
provide an override of `seekoff`.
+ Requirements for `seekoff` are in 27.7.1.3.
+ 27.7.1.3 - Standard has a roundabout way of saying `cur` is valid if only one
of `in` or `out` is specified (Condition 3).
+ `xend` is `egptr`/`epptr` (get area/put area).
+ `xbeg` is `eback`/`pbase` (get area/put area).
+ `xnext` is `gptr`/`pptr` (get area/put area).
+ `newoff` is a bit of a misnomer, the new offset into the stream is given by
`newoff + off`. `newoff` is really the start point.
+ You can set `gnext` with `setg`, but you can't do that with `setp` you have
to `pbump` by the new offset minus the current offset.
