boost17x: 1.77 -> 1.78

[TheBenCollins]

Description of changes

Added the December 2021 release of Boost, 1.78.0. This PR not only adds a new package, pkgs.boost178, but also moves the indirect package pkgs.boost17x to point to the new library. This later change is more substantive as there are many other derivations that depend on this meta-package.

Things done

• Built on platform(s)
  • [ x ] x86_64-linux
    I ran nix-build -A pkgs.boost178 . from the local clone of nixpkgs and got a proper result.

I was not able to run nixpkgs-review in a reasonable amount of time (was ongoing for many hours) so this check is left undone.

[ajs124]

Thanks for your contribution.
Some notes: This should go to staging, because it rebuilds >5k packages. Also, the commit messages don't follow CONTRIBUTING.md, I think.

For how to change the target branch to staging without spamming a bunch of people, maybe check #166472 or CONTRIBUTING.md, once it's merged.

[TheBenCollins]

@ajs124 I rebased onto staging as was requested. (I followed the directions in #166472). Let me know if there is anything else to work out.

[ajs124]

Thanks! The commit messages should follow the pattern outlined here. I know it's kind of pedantic, but it makes commit logs easier to read IMO.

[dotlambda]

superseded by #175694