Drop the Default bound on the peripheral traits

[rrbutani]

what

This was an early design decision we made that, while well-intentioned (the idea, IIRC, was to make the API easier to use; with this bound we'd always be able to provide a Sim::default() function), was probably ill-advised.

For embedded devices in particular this bound is challenging: most peripheral implementations for devices simply need to reference external state and requiring Default makes this impossible without resorting to statics or unsafe.

On hosted platforms, implementing Default on peripheral implementations (like the shims) is still desirable and we can still provide Default implementations on types like Interpreter and Sim by just making those impls contingent on a Default bounded Peripherals type.

steps

• remove the Default bound on the peripheral traits
• change impls higher up in the type hierarchy to require : Default as needed

where

branch: imp/drop-default-on-peripheral-traitß

open questions