formatting fixes and wording tweaks in the changelog
changing `ht-map` to return a list

Suggested by jfb on https://news.ycombinator.com/item?id=5613177
implementing `ht-each` and `ht-aeach`
removing redundant nconc in ht-map

nconc should have been nreverse here, but since iteration order for
hash tables is undefined anyway, there's no point
rewriting many functions in terms of ht-amap for clarity
documenting an API change
documenting installation
featuring the examples more prominently
mentioning that `ht-map` differs from `maphash`
mentioning ht-each and ht-aeach in the readme
added ht-contains-p
tweaking strapline in readme to match elisp source