1715 Drop array bound checking

[michaelhkay]

Fix #1715

Drops array bound checking from array:get, arrays-as-functions, and array lookup. Returns () instead of an error FOAY0001 when the index is out of bounds. This brings arrays and maps into closer alignment.

Drops the $fallback argument of array:get()

Adds a new function array:get-if-present() which replicates the old behaviour of array:get().

Functions such as array:put, array:replace, array:insert-before, array:head, array:tail continue to perform bound checking.

[ChristianGruen]

Functions such as array:put, array:replace, array:insert-before, array:head, array:tail continue to perform bound checking.

To improve symmetry with sequences, I think we should get rid of the checks for all read function, including array:head and array:tail, to ensure that expressions like $array(1) and array:head($array) can be mutually rewritten (both by users and optimizers). But I agree it’s a good idea to keep the checks for updates.