checker upgrades

[pearsonca]

Want more info in error messages:

• in error message: show calling scope / checked function name (i.e. indicate helloworld in example errors)
• in error message: show value / variable suppled to checked function (i.e. 1, c("Alice","Bob"), and "")
• consolidate checker / checker_against backend - should be able to seek to location in sys.call for the gsub, then take some chunk after that as the other variables
• robustify get_pipe... to account for users not defining their functions with check_... names
• provide a check_(x, ...) function that will use other checkers