Update input checks to use rlang type checking

[thomasp85]

This is an issue that is mostly about code sanitising. The code base is littered with checks for e.g. various types of scalars etc for which rlang now provides good self-documenting checks.

[teunbrand]

Another pattern that is very common and could be wrapped is the following:

if (!input_has_class(input)) {
  abort("Input should have class xyz")
}