distro-build/assemble-site: merge native libraries
Support require/typed clauses in require/typed/provide

Closes PR 13901
Fix reading of config.rktd file when in strange readtable.
Move json tests to racket-test pkg.
Don't run special sandbox tests when the relevant libs are not available.
Create `unstable-debug-lib`, `unstable-pretty-lib`,  `unstable-flonum-lib` packages.
Add `scribble-text-lib` package.
Add more correct dependencies for `racket-test`.
Fix unneeded R6RS dependency.
Configuration for Travis-CI.