Add with_bytes / with_string method to messages

[mjhouse]

These methods should allow users to bulk-add data words up to 64 bytes total, or 32 data words. the with_bytes, set_bytes and add_bytes methods would have the following signatures:

pub fn with_bytes(mut self, data: &[8]) -> Result<Self>;
pub fn set_bytes(&mut self, data: &[8]) -> Result<usize>;
pub fn add_bytes(&mut self, data: &[8]) -> Result<usize>;

The with_string, set_string, and add_string methods would be similar but take a &str param.