merge 2.12.x onto 2.13.x

[SethTisue]

it's been a while, so it's time:

% git hist --merges origin/2.12.x ^origin/2.13.x      
* 90c94cc2ea - (origin/HEAD, origin/2.12.x) Merge pull request #6949 from som-snytt/issue/t10956-fix (20 hours ago) <Seth Tisue>
* 9a8328e468 - Merge pull request #6933 from adriaanm/t11020 (2 days ago) <Adriaan Moors>
* f552875c17 - Merge pull request #6919 from retronym/ticket/10858 (7 days ago) <Lukas Rytz>
* cf4fb702b1 - Merge pull request #6882 from jhnaldo/2.12.x (13 days ago) <Julien Richard-Foy>
* cbfcdb6339 - Merge pull request #6865 from som-snytt/issue/10956-backport (2 weeks ago) <Dale Wijnand>
* 0d4624123a - Merge pull request #6696 from rorygraves/mike/2.12.x_rangepos_pr (2 weeks ago) <Jason Zaugg>
* 21c593e818 - Merge pull request #6754 from hrhino/topic/rangepos-no-validate_2.12 (2 weeks ago) <Jason Zaugg>
* d322514e16 - Merge pull request #6846 from hrhino/t9529 (3 weeks ago) <Lukas Rytz>
* b29d891b8f - Merge pull request #6850 from NthPortal/bug#10747/PR (3 weeks ago) <Lukas Rytz>
* f29c55b424 - Merge pull request #6848 from hrhino/faster/getParts (3 weeks ago) <Jason Zaugg>
* ec21aad47c - Merge pull request #6828 from retronym/ticket/10703 (4 weeks ago) <Lukas Rytz>
* aa9f79f03a - Merge pull request #6788 from lrytz/b10487 (5 weeks ago) <Lukas Rytz>
* 5622f5d4a2 - Merge pull request #6763 from hrhino/t10937 (5 weeks ago) <Lukas Rytz>

6828, 6754, 6865, and 6949 are backports which can be merged with -sours

most merge conflicts were easy/obvious, but I ran into conflicts I didn't know how to resolve in scala/scala#6848 (@hrhino) and scala/scala#6696 (@rorygraves), so someone else on the team should take this on

[SethTisue]

(on scala/scala#6848 I asked @hrhino if he'd submit a forward-port PR)

[lrytz]

scala/scala#7009