New function alias: env()

[kykyi]

In relation to #1278, this PR adds a new env function which acts as an alias to both env_var and env_var_or_default:

env_var('x') => env('x')
env_var_or_default('x', 42) => env('x', 42)

To make such a function signature possible, a new UnaryOpt enum variant was added which accounts for the majority of the touched files!

[casey]

Thanks for the PR! It looks like the env function will silently ignore excess arguments, which isn't great. How about something like:

  UnaryOpt(fn(&FunctionContext, &str, Option<&str>) -> Result<String, String>),

Which accepts only one or two arguments?

[kykyi]

Good call @casey! I have updated to introduce UnaryOpt 👌

[kykyi]

These tests break the function_argument_<variant> convention by adding a too-many and a too-few test, unsure if this is what you want or should just have the single test.

[kykyi]

remove(0) to grab from the front of the list, then a pop on the last element, happy to hear a prettier way 😅

[casey]

Nice, merged!