Compact hash representations of arbitrary R objects

The digest package provides a principal function `digest()`

for the creation of hash digests of arbitrary R objects (using the md5, sha-1, sha-256, crc32, xxhash and murmurhash algorithms) permitting easy comparison of R language objects.

As R can serialize any object, we can run `digest()`

on any object:

```
R> library(digest)
R> digest(trees)
[1] "12412cbfa6629c5c80029209b2717f08"
R> digest(lm(log(Height) ~ log(Girth), data=trees))
[1] "e25b62de327d079b3ccb98f3e96987b1"
R> digest(summary(lm(log(Height) ~ log(Girth), data=trees)))
[1] "86c8c979ee41a09006949e2ad95feb41"
R>
```

By using the hash sum, which is very likely to be unique, to identify an underlying object or calculation, one can easily caching strategies for which the digest package is somewhat widely used.

A small number of additional functions is available:

`sha1()`

for numerally stable hashsums,`hmac()`

for hashed message authentication codes based on a key,`AES()`

for Advanced Encryption Standard block ciphers.

Please note that this package is not meant to be deployed for cryptographic purposes for which more comprehensive (and widely tested) libraries such as OpenSSL should be used.

Dirk Eddelbuettel, with contributions by Antoine Lucas, Jarek Tuszynski, Henrik Bengtsson, Simon Urbanek, Mario Frasca, Bryan Lewis, Murray Stokely, Hannes Muehleisen, Duncan Murdoch, Jim Hester, Wush Wu, Qiang Kou, Thierry Onkelinx, Michel Lang, Viliam Simko, Kurt Hornik and Radford Neal.

GPL (>= 2)