Nonescapable stdlib primitives (#2657)

* Initial version of the pitch text

* Proofreading edits

* Resolve FIXMEs that I left in by accident

* Add link to forum pitch; update TOC

* Dedupe sections on lifetime management

* Remove unsafeBitCast generalization for now

The proposed variant is much too limited, and it also introduces a new axis of unsafety that may better be expressed with dedicated API.

We do not see any _immediate_ need for bit casting between escapable/nonescapable types, so it seems we can safely defer this work.

* Reword section on ObjectIdentifier; add note on ==/!= operators over metatypes

* Add metatype comparisons; take back Optional.unsafelyUnwrapped; minor edits

- Add generalizations for `==`/`!=` over existential metatypes
- Update ObjectIdentifier.init to use the right definition now that it works
- Take out the generalization of `Optional.unsafelyUnwrapped` due to issues with C++ interop. (I expect we’ll fix this in the next proposal in this series.)
- Add explicit (illustrative) attributes to describe the lifetime semantics of generalized API.
- General copy editing

* One more round of updates

- Correct number of SE-0437 (!)
- Provisionally reintroduce `Optional.unsafelyUnwrapped`. (We trust the interop issue can be fixed soon.)
- Add explicit sections in Detailed Design describing inferred lifetime semantics for enum operations
- Reword the introduction of infinite lifetimes
- More copy editing

Author: lorentey