Simplify virtual store

[robin-nitrokey]

This patch uses the simplified Store trait to also simplify the store
implementation used by the virt module: Instead of using references to
static storages and filesystems protected by a mutex, we can now just
use separate non-static instances for every test.

The only downside is that we can no longer provide raw access to the
IFS via the StoreProvider::ifs function, so we cannot run the
provisioner app with this platform implementation. But I think this is
an acceptable tradeoff.

[robin-nitrokey]

With trussed-dev/littlefs2#98, we can use Box<dyn DynStorageAlloc> instead of type parameters for the three storage types. So I’ve removed the StoreProvider trait. Callers can either use StoreConfig::ram() to quickly setup in-memory storage or compose their own StoreConfig (e. g. in the usbip runner). As the store setup is now so easy and we use custom wrapper functions in most crates anyway, I’ve also removed the with_{fs,ram}_client{s,} helper functions to keep things simple.

[robin-nitrokey]

This reduces the runtime for the fido-authenticator integration tests from 80 s to 33 s for me.