Review all API changes and deprecations before cutting Mill `1.0.0`

[lefou]

Ideally, we can backport all deprecations into 0.12.x and remove them for Mill 0.13.0. All that's needed then, is to tell users to first upgrade to latest Mill 0.12.x before upgrading to 0.13.

[lihaoyi]

Another important thing to do here is take a pass at privateizing everything that can be made private, to allow easier evolution later in the 0.13.x cycle

[lihaoyi]

Closing this since 1.0.0 is out