add ! to functions mutating a data frame

[ralsei]

Pretty much as the title suggests. Old function names are preserved, for compatibility's sake.

There doesn't appear to be a contract-out or provide/contract form that helps with regards to duplicating the contract at the end.

[alex-hhh]

I like this change and it will make the names more consistent, however, I would like some changes:

• The function definitions themselves should have the "!" in their name (e.g. (define (df-add-series! ...) ...)
• The Scribble documentation should only mention the "!" variant, since we deprecate the old names
• The old names should be in a separate provide block with the rename (so they can be disabled with a single #; comment):

(provide
 (rename df-put-property! df-put-property (-> data-frame? symbol? any/c any/c))
 (rename df-del-property! df-del-property (-> data-frame? symbol? any/c))
 ;; .. and the rest
)

• update the names in the tests themselves as well (this would be easiest to do by commenting out the provide block above and getting the tests to run without errors.

[ralsei]

Updated function names internally, and all tests passed on my machine with the provide/contract blocks at the end of df.rkt and statistics.rkt commented out. YMMV, but I hope not.

Do let me know if I missed anything.

[alex-hhh]

Thanks for making these changes, I merged the PR.