Move gc-assert stuff to EXTENSIONS package

Move GET-GC-ASSERTIONS and SET-GC-ASSERTIONS to the EXTENSIONS package
since that's where *GC-VERBOSE* is.
2 jobs for issue-69-compile-in-gc-assert in 17 minutes and 6 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #436
linux
linux-runner

00:17:05

passed #437
osx
osx-runner

00:08:07