Type checkers

[jennybc]

Basically moving the writing and guidance part of r-lib/usethis#1692 here, now that our upkeep checklist encourages the use of usethis::use_standalone("r-lib/rlang", "types-check")

... how to write your own check functions:

• Should have arg and call arguments.
• When type checking, always report what the object actually is.

[jennybc]

I am also going to close an issue in R Packages that is best covered here.

hadley/r-pkgs#803