lib: introduce digest library

[sebastianertz]

Introduce a new "digest" library which provides bindings for libmd operations. #112

md2(str);
md2_file(path);
md4(str);
md4_file(path);
md5(str);
md5_file(path);
sha1(str);
sha1_file(path);
sha256(str);
sha256_file(path);
sha384(str);
sha384_file(path);
sha512(str);
sha512_file(path);

[jow-]

Merged, thanks! Sorry for the long delay.