enable object_name_linter and apply it

[MichaelChirico]

Closes #597

[lintr-bot]

R/utils.R:1:1: style: Variable and function name style should be snake_case.

​`%||%` <- function(x, y) {
^~~~~~

R/utils.R:9:1: style: Variable and function name style should be snake_case.

​`%==%` <- function(x, y) {
^~~~~~

R/utils.R:13:1: style: Variable and function name style should be snake_case.

​`%!=%` <- function(x, y) {
^~~~~~

R/utils.R:17:1: style: Variable and function name style should be snake_case.

​"%:::%" <- function(p, f) {
^~~~~~~

R/zzz.R:185:1: style: Variable and function name style should be snake_case.

​.onLoad <- function(libname, pkgname) {
^~~~~~~

[MichaelChirico]

For covr: everything is lines that will be nocov'd in #629, so let's re-run after merging that