CXX-3232 add bsoncxx v1 declarations

[eramongodb]

Resolves CXX-3232. Followup to https://github.com/mongodb/mongo-cxx-driver/pull/1318, https://github.com/mongodb/mongo-cxx-driver/pull/1401, and https://github.com/mongodb/mongo-cxx-driver/pull/1402.

This is 2 out of an estimated 7 major PRs which in total are expected to resolve CXX-2745.

• [x] Top-Level Dependencies (CXX-3239)
• [ ] bsoncxx v1 Interfaces (CXX-3232) (<-- you are here)
• [ ] bsoncxx v1 Implementations (CXX-3233)
• [ ] bsoncxx v_noabi Refactor (CXX-3234)
• [ ] mongocxx v1 Interfaces (CXX-3236)
• [ ] mongocxx v1 Implementations (CXX-3237)
• [ ] mongocxx v_noabi Refactor (CXX-3238)

This PR introduces the very first set of v1 ABI exported symbols. 🎉

... only because MSVC complains otherwise. The rest will come as planned in CXX-3233 (impls).

---

Due to the volume of changes relative to the equivalent v_noabi interfaces, the bulk of relative changelog (v_noabi -> v1) and rationale for design decisions are documented inline as GitHub comments (coincidentally, exactly 100 comments in total).

In summary, notable changes include:

• General Notes
  • Nearly all preconditions are now documented and well-defined to instead throw an exception or yield an "invalid" state.
  • Narrowing casts and discarded return values are now thoroughly accounted for as new error codes.
  • All potentially-thrown exceptions are now documented and include the list of error codes which may be thrown.
• v1::error
  • All exceptions thrown by library components have a corresponding error code enumeration declared in v1::error with corresponding error category declared in v1::error::category.
  • Error conditions source and type are provided as the primary user-facing error code query mechanism when component-specific error codes are not necessary.
• v1::types::b_*
  • All types are now consistently non-aggregate for list-initialization consistency.
  • All "single underlying value" types now consistently support implicit conversion to said value.
  • Multi-argument constructors are no longer explicit (always favored over value).
• v1::types::view
  • This type is now semitrivial (no user-provided non-default special member functions).
  • Conversion to this type is now favored over conversion to value when applicable.
• v1::types::value
  • All (non-default) constructors are now explicit (b_* is always favored) so either b_* or view (cheap) is always preferred to value (expensive) when applicable.
  • v1::types::id constructors are removed in favor of b_* and converting constructors.
  • "Inline PIMPL" avoids redundant allocation while preserving encapsulation.
• v1::element::view
  • The "invalid" state is now clarified and thoroughly documented.
  • "Inline PIMPL" leaves room for an alternative bool-based approach to "missing field" diagnostics (CXX-2120).
  • Although not semitrivial, the class is still nothrow copyable.
  • Equality comparison is defined as "same field in same BSON binary data". Relational comparison is also possible but deferred for now.
• v1::document::view
  • Now supports an "invalid" state without underlying BSON binary data.
• v1::array::view
  • Now (nearly) completely defined in terms of v1::document::view.
  • API is extended for better parity with v1::document::view (CXX-2118).
• v1::document::value
  • Now default-constructible (CXX-939), equivalent to an "invalid" v1::document::view.
  • deleter_type is now a std::function<T> instead of a function pointer.
  • Customization points for to_bson and from_bson are now properly constrained.
• v1::array::value
  • Now (nearly) completely defined in terms of v1::document::value.
  • API is extended for better parity with v1::document::value (CXX-2118).