Update matrix/array handling

[leeper]

For consistency with: https://github.com/chaselgrove/python-unf and associated discussions.

[leeper]

Some test cases:

> UNF::unf6(1:8)$formatted
[1] "UNF6:EOs7/j65kxOaQ+7pcXc3Ow=="
> UNF::unf6(array(1:8, dim = c(4,2)))$formatted
[1] "UNF6:EOs7/j65kxOaQ+7pcXc3Ow=="
> UNF::unf6(array(1:8, dim = c(2,4)))$formatted
[1] "UNF6:EOs7/j65kxOaQ+7pcXc3Ow=="
> UNF::unf6(array(1:8, dim = c(2,2,2)))$formatted
[1] "UNF6:EOs7/j65kxOaQ+7pcXc3Ow=="