Support for long vectors

[richfitz]

Currently (v1.2.0) sodium::data_encrypt is limited to encrypting vectors of length 2^31-17 or less. There are two slightly separate issues here:

library(sodium)
key <- sha256(charToRaw("This is a secret passphrase"))
cipher <- data_encrypt(raw(2^31 - 17), key) # works fine
cipher <- data_encrypt(raw(2^31 - 16), key) # integer overflow
## Error in data_encrypt(raw(2^31 - 16), key) : 
##   negative length vectors are not allowed
cipher <- data_encrypt(raw(2^31), key) # no support for long vectors
## Error in data_encrypt(raw(2^31), key) : 
##  long vectors not supported yet: memory.c:3887

This PR makes minor changes to the length access and storage to prevent this, following the API in R-ints where we use XLENGTH and type R_xlen_t which should be portable.

I've fairly blindly added these to the "large" components - i.e., messages and ciphertexts, but not to keys and nonces.

The package does not currently have tests, but current examples work with these changes.

[jeroen]

Thanks!