improves fall-back to native crypto routines on macOS (#31)

remove calls to SETLENGTH() and copy defensively.

fixed PKI.sign.tar() and PKI.verify.tar() mis-detecting
tar-balls with xz (lzma) compression. Also the detection
no longer fails in UTF-8 locales on systems where R
attempts to interpret the magic bytes as UTF-8. (#29)

added PKI.genpass() for random password generation

minor cleanup of casts to avoid spurious warnings

add support for large vector digests