Updating will management

[ryan-summers]

This PR refactors the will temporary storage such that the will is serialized at configuration time into a user-provided buffer. This means that the Will message itself does not need to live very long.

Pros and Cons:

• User has to provide yet another buffer at config time
• Libraries can leverage will memory provided by the user to construct customized wills.
• Will serialization only ever happens once

[ryan-summers]

Another idea that we could do is have the user provide a single buffer during the Config builder. Then, when we want to serialize the will, we split the buffer to take part of it for the will.

Then, when we go to make RX/TX buffers, we can split the remainder down the middle to create the RX/TX buffers. This way. the user just has to give us a single chunk of memory that we use for everything.

We could even expose configuration features in Config to let the user specify the min/max RX/TX buffer sizes if we wanted.

[ryan-summers]

I spawned #146 to refactor, seems a bit of a bigger change than this PR was intending.