Towards v2.0.0

[fingolfin]

This PR is meant to collect a list of changes for v2.0.0 (either by referencing other PRs or issues to be fixed, or by including the changes directly), and also to prepare release notes.

TODO:

• PR remove @deprecate on Base.Dict (type piracy) #114
• PR Change pop! for LittleDict behavior #143
• ...
• add a "migration guide" to the manual that explains for each breaking change how it might manifest and what one can do to adapt the code (ideally in a way that works both in v1 and v2 ; this should also be made clear to the reader)
• prepare an announcement of the new version to be posted on Julia Discourse
• prepare release notes (see below

Draft of release notes for JuliaRegistrator bot:

Release notes:

## Breaking changes

- blah