optimizer: more optimizations for begin0

Move begin0 inside begin, for example
(begin0 (begin X Y) Z) ==> (begin X (begin0 Y Z))
Try to replace more begin0 with begin when the first expression is movable
Drop the begin0 when it has only one non omitable expression that preserves
the continuation marks.
simplify treatmenet of `begin0` and discarding expressions

Since `begin0` at the bytecode level always evaluates an initial
expression in non-tail position, we don't have to work so hard
to ensure that an extra expression sticks around.
restore non-places parallel build

The build protocol had evolved to include paths in messages passed
between places/processes, and that doesn't work with processes.
fix a problem mixing JIT and non-JIT code

Crashes the "optimize.rktl" test suite when the JIT supported but
disabled, because that test suite re-enables the JIT.
fix continuation reuse in non-JIT mode

The continuation mark to generate stack traces interfered with the
detection of equivalent continuations.
More hyperlinks between info.rkt docs

This adds links between most of the pages that
are hits for "info.rkt" in the doc search so that
it's easier to find the field specifications.
pkg-directory: add `#:cache` argument

The cache enables multiple calls to `pkg-directory` to load
installed-package information only once.
makefile: remove lingering references to special native packages
package "info.rkt": define `distribution-preference`

For now, the client is "distro-build".
make: always `raco setup` in default target

Use `--no-setup` when installing packages to avoid a
definitely-reundant setup.
Fix external packages built on drdr.
Remove release announcements.

These are now in the `racket/racket-lang-org` repository.
Split `meta/images/taking-screenshots` from the repository.

This is now in the `racket/taking-screenshots` repository.
Remove `mkheart.rkt`, which has been moved to the `racket/drracket`
repository.
Move unix installer test to the "racket-test" pkg.
Now that this isn't an actual package, it doesn't need metadata.
raco pkg update: make update to non-clone suggest more non-clone conversions

In other words, suggestion conversions in the non-clone direction the
same as conversions in the clone direction. As a way of disambiguation
the right direction, the non-clone direction is only suggested for
sharing that is immediately discovered from the command-line arguments
(as opposed to sharing that becomes apparent as other packages are
updated or installed via dependencies).
raco pkg: change default answer on clone-sharing conversion to "Y"
raco pkg update: allow `--update-deps` with a linked package

The linked package cannot be updated, but maybe its dependencies
can, so just alert the user that the linked package is skipped
instead of rejecting the request.
raco pkg: fix path problems with updating a clone-linked package