Use 0o prefix for octal literals, and gofumpt all files

[bitfield]

Since Go 1.13, octal literals can be written using an 0o prefix, instead of the (slightly) ambiguous 0. Indeed, gofumpt insists on this, and those of us who use gopls with gofumpt enabled have our octal literals rewritten for us automatically.

This commit applies that change, along with some other minor gofumpt-age, including adding or removing blank lines and reformatting doc comments.

[mvdan]

Goes without saying that I like these changes, but perhaps that isn't a proper approval 😉

[mvdan]

This feels low risk, so I'll go ahead and merge. @rogpeppe can always send a follow-up PR or a revert if he disagrees.