Promotes the four changelog.d fragments into a `## [0.7.0]` section:
sb-1g4q, sb-mmyj, sb-oje0 and sb-z80a, grouped Added 3 / Changed 3 /
Removed 2 / Fixed 1. Every fragment bullet is carried over byte for
byte; the prose written here is the lead paragraph, the `[0.7.0]` link
reference, and one Removed bullet.

That bullet covers sb-9t19, which shipped no fragment. It dropped the
toolbar's `inserting?` attribute, inert since 0.6.0 removed the "Cancel
insert" button but left the attribute declared. A host that kept
passing it compiled clean on 0.6.0 and now gets an undefined-attribute
warning, so the removal is one a caller can see.

Bumps the version to 0.7.0 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-084r