RFC: Bundle type

[thibault-martinez]

Rendered

[SuperFluffy]

Incorporated into #20.

While discussing the design of the Transaction type, see #3, we realized that a new transaction is not supposed to be create directly. Instead, when constructing a new message, the Bundle is the fundamental object one wants to crate, which in turn splits up its message into several Transactions. A Transaction thus becomes a very simple type. More complicated logic should thus be handled by the Bundle.

This makes it necessary to expand the Bundle RFC beyond what is proposed here.