clarify the word 'bitmap' in context of image tp
improve the performance of rotated or scaled (but not flipped) bitmap
drawing in 2htdp/image

closes PR 13895
Move most of the `net` collection out of the core.

- Most units and signatures from the `net` collection
  are now in `compatibility-lib`.
- Most of the actual libraries are in the `net-lib`
  package.
Copy many fewer files for xform.
Fix uses of 'mzscheme' in comments.
Remove most uses of `mzscheme` in the core.

Remaining are:
 - parts of unit200 that Matthew plans to remove.
 - the `mzscheme` implementation itself.

The implementation of `mzscheme` has been moved
to the `mzscheme` collection (from the `racket` and
`scheme` collections). The `scheme/mzscheme`
language, which was undocumented, has been removed.

This is slightly backwards-incompatible, because
the `xform` handling of precompiled headers now
evaluates code in a `racket/base`-like namespace,
instead of in a `mzscheme`-like namespace.
Move most of the `compiler` collection to `compiler-lib`.
fix variable-quoting conventions in top-level Makefile
distro-build/farm: fix `#:dist-base'
fixup for `mzscheme' -> `racket/base' conversion
fix build on version change
distro-build/farm: add `#:pull?' configuration
distro-build/farm: fix `#:pkgs' handling
distro-build/farm: improve break handling
raco pkg: fix handling of overlapping provided-package sets
change exports from the 'pkg' module so they are all prefixed with pkg-

(the goal is to avoid shadowing 'remove')
add transparent "color" to 2htdp/image.
Make struct: work at the REPL

Closes PR 11669
add a section to the image guide about rotation and centers
make doc-search URL configurable for installer builds

Also, add an initial-catalogs configuration to clients and
`farm' builds.