Look at changing permission adapters to not use weak linking

[marcpalmer]

Following the landing of workaround #145, we'd like to look at more sustainable solutions. Something like one of:

1. Microframeworks (more hassle for user, can link without tricks though, more relinking overhead, more git admin/release busywork)

2. App-side protocols that mimick the APIs required, assigning instances to e.g. Flint.contactStore so permission adapter can use them. We still can't use the real APIs inside FlintCore in this scenario

3. Swift compiler defines passed to Carthage/Cocoapods at build time to specify which modules are required e.g. OTHER_SWIFT_FLAGS=-DFLINT_CONTACTS -DFLINT_PHOTOS and we wrap all access and imports of such APIs with checks for those. Fiddly for users/config as you can't just check out the project and build it, but lets us use all the APIs.