Assembles the ten changelog.d fragments into a `## [0.5.0]` section,
grouped Added / Changed / Fixed, and deletes them. Bullets are copied
verbatim; changelog.d/README.md stays. The section opens with a lead
paragraph saying what the release is, and the link references at the
bottom of the file gain 0.5.0 along with the 0.4.0 and 0.3.0 entries
that were never written.

Moves `@version` and the compiler's `@compiler_version` to 0.5.0. The
compiler-version bump rides the release, as it has since 0.4.0;
compiler_test.exs asserts the constant against mix.exs, so no test
pins a literal.

Pins the README's install snippet at `~> 0.5`.

Not tagged and not published.

Refs: sb-dky