Util to determine the bytes consumed by a stream

[kammce]

Streams return the amount of data remaining, but do not give back information about how many bytes were consumed by the pipeline. This information is helpful as it can be used to shovel the data that was consumed into other parallel pipelines.

Currently:

auto remaining = input | fill | find | parse<unsigned>;
// We get how much is left of input, but we don't know how much of input was consumed by the pipeline

See libhal-google#8 for the previous discussion.