Add type for `this-language` to fix test

(also add a type for `all-languages`)
Fix indentation
fix string-constants
Add type for `dynamic-string-constants`
Adjust test for API coverage

Also Rackety
fix bug in DrRacket
raco pkg create: extend `--from-install` mode so it can unmove files

This adjustment to packing allows a native-library package to
be converted back from installed to installable.
raco setup: add `install-platform` to "info.rkt"

The `install-platform` value determines whether files are
copied to "lib" and "share" directories.
raco pkg {install,update,migrate}: add `--all-platforms`

The `--all-platforms` flag causes the package manager to follow all
package dependencies, even for dependencies that are specific to
platforms other than the current one.
declare platform for each native-library package
makefile fix for catalog-based installer build
remove bad `return`
make catalog-based installer build work

A catalog-based installer build (as opposed to an installer build
that uses "pkgs" and "native-pkgs") relies on the new `--all-platforms`
package-install flag and related support for re-packing installed
platform-specific packages.
Support gzip encoded HTTP responses
Refactoring.
Fix type error message pruning for tc-any-results.
Avoid swallowing errors.

This hid the fact that error pruning was failing.
Refactoring.
Add tests for error message pruning.
move unstable/2d out of the gui-lib pkg