proposal: make a bunch of stuff inlinable

### Current Situation

effectively none of the APIs in FHIRModels can be inlined by apps/packages using the FHIRModels package.
this is can become very painful, especially e.g. when creating a lot of `FHIRPrimitive`s where the underlying value is just a copy of the input (eg: `FHIRString`, `FHIRURI`, `FHIRDecimal`, etc.)

context: our application's primary use case for FHIRModels is creating Observations (and via that a bunch of other FHIR types), which then get JSON-encoded and uploaded to a server. (we also sometimes use the package to decode and inspect data, but that's a tiny minority of our usage.)
it's not uncommon for our app to create millions of FHIR samples per day; as a result the lack of inlinability does actually have a significant and easily noticable impact on our app's performance.

### Suggested Solution

i'd like to propose Apple consider marking as much of the FHIRModels package's public API as possible as `@inlinable`:
- all trivial operations on `FHIRPrimitive` and its companion types (`FHIRString`, `FHIRURI`, `FHIRDecimal`, `FHIRInteger`, etc)
- initializers
- equatable implementations
- other operations (eg: stuff like `Base64Binary.data(using:)`, `FHIRPrimitiveProtocol.extensions(for:)`, `Instant.asNSDate()`, etc)
- all of the `asFHIR{Type}Primitive()` extensions
- all initializers on `FHIRType`-derived classes
- including the convenience initializers; since they typically default all of their parameters to `nil`, my understanding is that the compiler would end up emitting code only for setting those properties that actually are passed into the initializer, but i'll need to double-check that)
- all trivial operations on `FHIRType` and its derived classed (eg: the `FHIRAbstractResource` hash and equality operations)

there are some things that probably don't make sense to inline (`ResourceProxy.get` comes to mind; i'm not sure how the compiler would handle that)