implement similar_varchar

[terite]

When using varchar on a field without max_length, the varchar replacer will throw the following exception:

TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

This PR makes the following changes:

• varchar replacer now returns a string of random characters between [1-255] in length.
• similar_varchar replacer returns a string of random characters the same length as the input value

[joeghodsi]

This has been open for a year. (lol) I am going to close it. If we ever need it again for some reason it can be found in the closed PRs list for this repo