Semantics of `position`

[jakobnissen]

The semantics of position are not clear, and depending on the choices, some implementations may be buggy. In particular:

1. Must position be an integer, or can it be a custom struct?
2. When having an IO struct that represents a compressed file being decompressed, is the position the position of the underlying file, or the decompressed data?
3. Must position be monotonically increasing as the IO reads/writes? Or, are IOs allowed to destroy previously read data, such that position may decrease?
4. Is position meaningful for non-seekable IOs?

The following behaviour may or may not be buggy

buf = PipeBuffer(; maxsize=8)
write(buf, "abcdefgh")
@assert position(buf) == 0
read(buf, 2)
@assert position(buf) == 2
write(buf, 2)
@assert position(buf) == 0

[vtjnash]

1. It should support simple math, usually corresponding to skip and seek behaviors, and the return value from write
2. It should be the uncompressed offset. The way data is stored should usually be opaque in the IO API (see 1)
   3, 4. A mark is required to force the position to have meaningful behaviors for a non-seekable IO