Concatenate.Strategy.Prepend should maybe be removed

[Jake-Shadle]

All of the tests and examples (and current usage at Embark) only uses the Append strategy, which is very easy and efficient since it's just an append to a heap buffer, however the Prepend strategy is not, as it must shift the entire packet contents first. Not sure if anyone uses this, but it seems as if Append is strictly better, so Prepend should either be removed, or at least come with a warning that using it will be less efficient.

[markmandel]

or at least come with a warning that using it will be less efficient.

I'd be inclined towards this approach. Someone will have some kind of legacy packet / constraint that requires prepend - so I figure there's no point in removing the feature.