Add several conversion, string, and logic functions (mostly from MS SQL Server)

[StevenHibble]

This is a loose collection of additions to the translations. There is one fix that I noticed while looking for missing function (nullif's base R translation). Here's what is added:

1. Conversion functions from SQL Server (TRY_CAST, CONVERT, and TRY_CONVERT) to complement CAST
2. String functions (STRING_AGG, REVERSE, REPLACE, CHARINDEX, and REPLICATE)
3. Logic functions (ISNULL, NVL, CHOOSE)

[StevenHibble]

It looks like there aren't tests for translations in general. I'm not sure how that would work, but I'd be happy to provide some.

[ianmcook]

Awesome, thank you @StevenHibble! I will take a closer look at this PR shortly.

Correct, I haven't yet added comprehensive tests of the function translations. If you'd like to give that a go, please do. You could add a test script named test-translations.R and create tests like the ones in test-parse_expression.R, calling parse_expression() but in a way that isolates specific function translations as much as possible. Or if you have ideas for how to do it better or more efficiently, go for it. Thanks!

[ianmcook]

P.S. The main reason that the codecov test coverage is not at or near 100% is that many of the function translations are currently not tested!

[ianmcook]

This looks great—thank you! I have just a few small change requests, and some questions and general discussion—please take a look at my comments. I'll wait to merge until after you push commits to address those (they should automatically get added to the PR).

[StevenHibble]

I'll try to make these changes over the weekend.

[ianmcook]

Thanks! And please don't feel any obligation to rush 🙂

[StevenHibble]

R CMD check did throw this warning:

checking for unstated dependencies in ‘tests’ ... WARNING
'::' or ':::' import not declared from: ‘dplyr’

[ianmcook]

To stop those unstated dependencies warnings, you'll need to call tidyverse functions by using str2lang(), just like you did in the translations you added for charindex() and replace().

[StevenHibble]

Thanks for the guidance. The references were in tests added for the NULLS FIRST/LAST commit.

[ianmcook]

Thank you @StevenHibble! I will review soon.

[ianmcook]

Sorry for the epic delay in merging this. Thanks again for your work on this!