Ensure all examples work and test file matches name of source file

examples for `full_argument_names` throw errors
```r
full_argument_names(expression(gsub(' ', '_', 'a b c')))
# Error in args(eval(x_fn[[1]])) : could not find function "a_b_c" 
```

This works though
```r
full_argument_names(parse(text = "gsub(' ', '_', 'a b c')")[[1]])
````