MINOR: vecpair: add new vector pair based data manipulation mechanisms

MINOR: vecpair: add new vector pair based data manipulation mechanisms

The buffers API defines both a storage layout and how to handle the
data. The storage is shared with the chunks API which only deals with
non-wrapping messages while buffers support wrapping both of the data
and of the free space. As such, most of the buffers code already makes
special cases of two parts in a buffer, the first one before wrapping
and the optional second one after the wrapping occurred.

The thing is, there are plenty of other places (e.g. rings) where the
code dealing with wrapping is desirable but with a different storage
layout. Let's export the existing buffer handling code related to
reading/writing wrapping data and make it work with arbitrary vector
pairs instead. This will handle wrapping and holes in messages if
desired, and it will be up to the caller to decide how its messages
are arranged and to pass the relevant ptr,len elements.

The code is limited to two vectors because this is sufficient to deal
with wrapping without making the code needlessly complex. I.e. this will
not reassemble an iovec. For vectors, since we already had the ist type,
there's no point inventing a new type, and it's even possible that over
time some callers will find benefits in using this unified API (i.e. no
NOP translation layer). It also allows to pass inputs as direct arguments
and outputs as pointers. Not only this is more efficient code-wise, but
it also avoids the accidental use of a wrong function. It was indeed
found that naming functions is even harder than with the buffer as the
notion of from/to is even fuzzier here.

The API will likely continue to evolve and some functions might get
renamed to more explicit ones over time to limit confusion. For now
the code provides anything needed to reset/create/fill/erase/read/peek
or measure vector pairs and to manipulate chars/blocks/varints to/from
there.