Make CHANGELOG.md adhere closer to the Keep A Changelog format

[JelteF]

Cleanup of #254

• The "Internal changes" sections under 0.7.1 and 0.9.0 have been removed - users of your library don't need (nor do they care) about internal details, so long as it works.
• The section for 0.99.4 was retitled to include [YANKED].
• Certain changes, such as the MSRV changes, or the change about all features except std being disabled by default, were reworded. This is either for clarity, or to make them fit better as changelog entries.