[docs] LibraryEvolution.rst: What kind of changes are ABI-safe?

Update the Library Evolution ("Resilience") doc with a listing for every
top-level declaration kind (...except operators and imports, TODO) that
describes what should be an ABI-compatible change between releases of a
binary library shipped independently of a client app (say, because it's
part of the OS). Restructure the existing discussion of "fragility
attributes" to describe how each of those capabilities is affected when,
say, marking a struct as "fixed-layout".

Still a work in progress, and if something unexpected catches your eye
I'd like to hear about it. Note also that this does /not/ describe how
Swift behaves today; we still make many assumptions about types that
will not be valid in a "resilient" world.

Author: jrose-apple