This PR translates `lean::mk_projections` into Lean, adding
`Lean.Meta.mkProjections`. It also puts `hasLooseBVarInExplicitDomain`
back in sync with the kernel version. Deletes
`src/library/constructions/projection.{h,cpp}`.
This PR introduces the central parallelism API for ensuring that helper
declarations can be generated lazily without duplicating work or
creating conflicts across threads.
This PR splits the environment used by the kernel from that used by the
elaborator, providing the foundation for tracking of asynchronously
elaborated declarations, which will exist as a concept only in the
latter.
Minor changes:
* kernel diagnostics are moved from an environment extension to a direct
environment as they are the only extension used directly by the kernel
* `initQuot` is moved from an environment header field to a direct
environment as it is the only header field used by the kernel; this also
makes the remaining header immutable after import
