Check and increase test coverage

[flying-sheep]

Options include

• https://github.com/jimhester/covr
• https://github.com/MangoTheCat/testCoverage

One predicate that every repr function for every format has to fullfill is

r <- repr_*(thing)

assert_true(is.null(r) || (is.character(r) || is.raw(r) && length(r) == 1L))

[jankatins]

Additionally from #5:

For vectors and so on: normal input and some edge cases: NULL, empty input, too long (ellipsis), unicode...