Redesign serializer system

[dktapps]

The current serializer system suffers from a few problems:

• No ability to reuse streams / memory
• Combined APIs make it easy to accidentally decode when encoding and vice versa
• Easy to call internal APIs that aren't supposed to be exposed to users
• Slow - several virtual calls during both decoding and encoding

Since ext-encoding integration would anyway cause some disruption to the API, we might as well take the opportunity to design a new API that can coexist with the old API, allowing PM5 to benefit from ext-encoding where code opts into the new system.