[Proposal] Opt-in Reflection metadata

[omax (Max Ovtsin)]

Apologies for the delay, I was on PTO
Here is the fixed pitch and updated prototype.

Pitch #2: Opt-in Reflection metadata

[Proposal] Opt-In Reflection metadata Proposal: SE-NNNN Authors: Max Ovtsin Review Manager: TBD Status: Pitch Implementation: apple/swift#34199 Pitch #1 Introduction Reflection can be a useful thing to create convenient and concise APIs for libraries. This proposal seeks to improve the safety of such APIs and to tackle the binary size problem by introducing a mechanism of selectively keeping reflection metadata only for types that need it and dead strip it for all others. Developers wi…