Adapt artifacts for changed build target directory

Install dependencies automatically

Install `sudoku-solver` dependencies

Add `libssl-dev` package

Seems to needed for TLS in newer Ubuntu TLS version.

Update version to 0.15.2

Update Racket version in `.build.yml`

Fix check-true and check-false

check-true and check-false consume only one argument.
If there are multiple things to test, they must be in separate expressions.

Increase version number to 0.15.1

There are no functional changes for the solver, only changes in the
build process.

Use `raco-exe-multitarget` for compilation

Replace the multi-target compilation code in the makefile with an
invocaton of `raco exe-multitarget`.

Use `al2-test-runner` for testing

Remove removal of `compiled` directories

This seems to be no longer needed for Racket 8.3 and recent versions of
raco cross.

Explain how to render dot files

Add `.PHONY` annotations

Add target `copy-to-releases`

Copy precompiled binaries to my releases website directory. This only
copies files locally. None are actually uploaded. This is left to a
Makefile in the `releases` directory.

Remove done todo items

Patch for version 0.15.0

Patch `.build.yml` to generate new version 0.15.0

Build with Racket 8.3

Update next version to 0.15.0

Clean up `README.md`

In particular, use distinct "Overview" and "Installation" sections.