New leftpad and rightpad DSL functions

[johnkerl]

Context: #1153 and #1166

Examples:

$ echo x=1234567 | mlr put '$x=leftpad($x, 10, "*")'
x=***1234567

$ echo x=1234567 | mlr put '$x=rightpad($x, 10, "*")'
x=1234567***

$ mlr help function leftpad
leftpad  (class=string #args=3) Left-pads first argument to at most the specified length (second, integer argument) using specified pad value (third, string argument). If the first argument is not a string, it will be stringified first.
Examples:
leftpad("abcdefg", 10 , "*") gives "***abcdefg".
leftpad("abcdefg", 10 , "XY") gives "XYabcdefg".
leftpad("1234567", 10 , "0") gives "0001234567".

$ mlr help function rightpad
rightpad  (class=string #args=3) Right-pads first argument to at most the specified length (second, integer argument) using specified pad value (third, string argument). If the first argument is not a string, it will be stringified first.
Examples:
rightpad("abcdefg", 10 , "*") gives "abcdefg***".
rightpad("abcdefg", 10 , "XY") gives "abcdefgXY".
rightpad("1234567", 10 , "0") gives "1234567000".

Unit-test cases to be uploaded next.

[derekmahar]

Are the results in these examples from above correct? I think you may have swapped the results.

leftpad("1234567", 10 , "0") gives "1234567000".
rightpad("1234567", 10 , "0") gives "0001234567".

[johnkerl]

@derekmahar typo indeed, thanks!