expand this array by `n` elements and call `filler` to set these elements.
This is useful if the new elements will not be added in order or if the new elements need to be moved around, e.g., for sorting.
`filler` will be run with an instanted instance of `container.expanding T` that will permit to set the new elements. During the execution of `filler` elements that have been set already (i.e., indices <length or those for which `set []` or `put` was called) maybe be read using `get` or `index []`.
`filler` may set elements repeatedly, but it must set all elements for the new indices `length`..`length+n-1` before it returns.
This is useful if the new elements will not be added in order or if the new
elements need to be moved around, e.g., for sorting.
`filler` will be run with an instanted instance of `container.expanding T`
that will permit to set the new elements. During the execution of `filler`
elements that have been set already (i.e., indices <length or those for
which `set []` or `put` was called) maybe be read using `get` or `index []`.
`filler` may set elements repeatedly, but it must set all elements for the
new indices `length`..`length+n-1` before it returns.
expand is not thread-safe.