This is the copying version of array/base/fill (which mutates) and a generalization of array/base/with (which only sets a single element).

The naive approach would be to simply copy (using array/base/slice) an input array (similar to array/base/with) and then overwrite the specified fill region. This should be fine, even if less optimal for array-like objects lacking a #.slice method.

The alternative would be to allocate a new array, copy values from the original array up until start, fill the specified region, and then copy values from the end index to the end of the original array. In principle, this approach would be better. In practice, it is likely to be less efficient, as built-in array slice operations are likely to use memcpy which is going to be faster than anything we do in userland.

Similar to array/base/with, we can provide an assign API. Accordingly, the proposed signatures:

toFilled( x, value, start, end );
toFilled.assign( x, value, start, end, out, strideOut, offsetOut );