Get rid of Sized requirements for inner wrapper types behind references

v0.17.2

feat(wrappers): get rid of Sized requirements for types behind refere…

Up- and Down-gradable APIs

Merge pull request #2 from andrewsonin/feat/up-and-downgradable-api

feat(up-and-downgradable-api): implement {Up/Down}gradableReadApi

Guarded Target: makes the deref target an associated type back

• feat(guarded-target): introduces GuardedTarget, makes the deref target an associated type back

Separate APIs: Deref target as generic parameter, not associated type

• feat(separate-apis): makes the deref target a generic parameter, not associated type

API Wrappers

• feat(wrappers): replace AutoApis with wrapper structs

Separate APIs: ReadApi and WriteApi

• feat(separate-apis): add ReadApi and WriteApi
• feat(separate-apis): add auto implementor traits AutoReadApi and AutoWriteApi

Initial release

• Add ReadWriteApi, TrivialReadWriteApi traits
• RwLock implementations support parking_lot = "0.12"