Features from `pluck()`

[GregYannes]

The Index() function is inspired by pluck() in the R language. Now pluck_exists() will test for existence, and pluck() itself will return a .default rather than throw an error:

# Test for existence...
pluck_exists(x, ...)

# ...and extract it (or default).
pluck(x, ..., .default = NULL)

Rather than throw an error, should our Index*() (and Arr_Index*()) families return a custom default like Null? And should we augment them with a convenience function like Index_Exists*() (and Arr_Exists*()?

Warning

Such a default argument would complicate the conversion (#2) of Arr_Index*() into a property. If it is present in the Get signature, it must be present in the Let/Set signatures—yet that is precisely where the value argument should go!