Merge block-sys into block2

`block-sys` doesn't actually build a library, it only links to it, so there is no real need for a build script, nor for the `links` key.

Additionally, it should actually only expose the six public symbols, and in that case, it just seems wasteful to have multiple crates.

This _does_ remove:
1. The ability to override the linking. This was brittle already, and is probably better handled by dedicated tools / by modifying the binaries after they're compiled / by inserting extra linker commands.
2. The `DEP_BLOCK_0_2_CC_ARGS` environment variable exposed to downstream build scripts. This was not really that useful, usually you want to know exactly what flags you're passing to your compiler, and the standard `-fblocks` flag is enough.

.github/workflows/ci.yml

@@ -38,7 +38,6 @@ env:
   #
   # This excludes `header-translator`, `test-assembly`, `tests` and `test-ui`.
   PUBLIC_CRATES: >-
-    --package=block-sys
     --package=block2
     --package=icrate
     --package=objc-sys
@@ -780,10 +779,8 @@ jobs:
     - lint
 
     env:
-      # `compiler-rt` is only relevant for these crates
-      PUBLIC_CRATES: >-
-        --package=block-sys
-        --package=block2
+      # `compiler-rt` is only relevant for block2
+      PUBLIC_CRATES: --package=block2
 
     steps:
     - uses: actions/checkout@v3

CONTRIBUTING.md

@@ -59,7 +59,6 @@ Copy and fill out the following checklist into the release PR:
     - `objc2-proc-macros`
     - `objc-sys`
     - `objc2-encode`
-    - `block-sys`
     - `objc2`
     - `block2`
     - `icrate`