Add array copying functions to heap_lang

Add array_copy_to (copy in-place to destination array) and array_clone
(copy to a freshly allocated array). The heap_lang spec and proof for
array_copy_to are inspired by
https://gitlab.mpi-sws.org/iris/lambda-rust/blob/3b4ae69fa3be1344245245bf05e5e80e790e064d/theories/lang/lib/memcpy.v.

Fixes #293.