Promotes the two changelog.d fragments into a `## [0.8.0]` section:
sb-2vqm and sb-e4r5, grouped Added 1 / Changed 1 / Removed 1. Every
fragment bullet is carried over byte for byte; the prose written here
is the lead paragraph and the `[0.8.0]` link reference.

sb-5lid shipped no fragment and needs none. It tidied ADR records, a
moduledoc and two prose precisions, which the changelog.d README
excludes by name as work a caller of the public API cannot see.

Bumps the version to 0.8.0 in mix.exs, in the compiler's
`@compiler_version`, and in the README's install pin. The compiler
identity assertion reads the version relationally, so no test moves.

Not tagged and not published: `mix hex.build` was run as proof and its
tarball deleted.

Refs: sb-j4ca