Split icrate into one crate per framework

Additionally:
- Add helper module `objc2::__framework_prelude` with common imports.
- Make `block2` an optional dependency.
- Make most framework crates except `objc2-foundation` optional.
- Move framework changelog to `objc2::topics::about_generated::changelog`.
- Add full list of crates in `objc2::topics::about_generated::list`.
- Document cargo features in `objc2::topics::about_generated::cargo_features`.

.gitmodules

@@ -1,6 +1,6 @@
-[submodule "crates/icrate/src/generated"]
-	path = crates/icrate/src/generated
-	url = https://github.com/madsmtm/icrate-generated.git
+[submodule "generated"]
+	path = generated
+	url = https://github.com/madsmtm/objc2-generated.git
 	# We recommend a shallow clone here since there are a lot of generated
 	# files in history, and I suspect most contributors won't need to look at
 	# the history of these.

CONTRIBUTING.md

@@ -5,10 +5,10 @@ Thank you for your interest in contributing! There are many ways to contribute a
 
 ## `git` submodules
 
-The `icrate` crate contains a submodule pointing to
-[`madsmtm/icrate-generated`]. This exists to keep the primary repository free
-from the clutter inherently associated with storing generated files in `git`,
-while still allowing change-tracking of said files.
+The framework crates uses a submodule pointing to [`madsmtm/objc2-generated`].
+This exists to keep the primary repository free from the clutter inherently
+associated with storing generated files in `git`, while still allowing
+change-tracking of said files.
 
 You should consider whether you want a shallow or a full checkout of the
 generated files - the default is to do a shallow clone, since the repository
@@ -28,15 +28,15 @@ See the excellent `git` [documentation on submodules][submodule-docs], or the
 manpage for [`git-submodule`] for further details on how submodules work.
 
 
-[`madsmtm/icrate-generated`]: https://github.com/madsmtm/icrate-generated
+[`madsmtm/objc2-generated`]: https://github.com/madsmtm/objc2-generated
 [submodule-docs]: https://git-scm.com/docs/gitsubmodules
 [`git-submodule`]: https://git-scm.com/docs/git-submodule
 
 
 ### Updating the `git` submodule
 
 When making changes to `header-translator`, you must update the `git`
-submodule manually and push your changes to a fork of `icrate-generated` (you
+submodule manually and push your changes to a fork of `objc2-generated` (you
 do not need to submit a PR to that repo).
 
 We try to maintain a linear history in that repo, in a fashion such that every
@@ -61,7 +61,7 @@ Copy and fill out the following checklist into the release PR:
     - `objc2-encode`
     - `objc2`
     - `block2`
-    - `icrate`
+    - Framework crates
 - Local tests have been run (see `helper-scripts/test-local.fish`):
     - [ ] macOS 10.14.6 32bit
     - [ ] iOS 9.3.6, 1st generation iPad Mini