raco pkg update: fix reoprting for `--all` in a scope without packages
raco pkg: fix tests for change `raco pkg show` output
raco pkg update: change printout to avoid alarming "Removing..."

Changed "Removing" to "Uninstalling to prepare re-install of".
Increase SCHEME_PRIM_OPT_TYPE_SHIFT

Otherwise, two optimization flags collide:

 SCHEME_PRIM_ALWAYS_ESCAPES = SCHEME_PRIM_PRODUCES_FLONUM = 8192
Mark immutable? as omitable
fix native-stack caching with libunwind

The implementation of caching stack-trace information in the
stack didn't work right in libunwind mode, with the result that
`(current-continuatiom-marks)` took O(N) time for a continuation
of size N, when it should be amortized constant time.
avoid a compiler warning
Rename racket-extra-test to racket-test-extra.
Split the core racket tests into the "racket-test-core" pkg.

This supports running the core tests on systems which can't run
or don't want to install all the dependencies of the "racket-test"
package.