[Pitch] Runtime enumeration of custom sections

SE-0492 gave libraries the writing half of binary-section placement and listed the reading half, "a unified runtime API for reading section contents across platforms and modules", as a future direction. This pitch is that follow-on: one runtime API that returns a named section's contents from every loaded image on every object format, with an observer for images loaded later. The API is prototyped as a library at swift-sections, which is what a library can do from outside the runtime and where ...