lib: add some list utilities

Adds `unfold` for constructing a list from a generating function, and
adds `range` for constructing a range of numbers.

Signed-off-by: Edward Pierzchalski <ed.pierzchalski@data61.csiro.au>

licenses: convert license tags to SPDX

lib: Add `find_index`.

lib: don't extend core signatures.

Just because we *can* extend the core SML `List` signature, that doesn't
mean we *should*. It's a neat trick, but it makes it harder to find uses
of the new modules, and obfuscates definitions for very little gain.