Added split_read() function to be able to read all commited data at once

[Sympatron]

Went ahead and implemented the idea I had in #76 myself.
There are 3 is 1 TODOs left:
1/2: I wrapped the second buffer in an Option, but that means I can't easily return it as Option<&mut [u8]> or Option<&'static [u8]> (cannot move out of 'self.buf2'')
3: Since I don't really understand this comment, I am not sure if this is sound the way I implemented it. I think it is, but this comment irritates me.

[Sympatron]

Maybe it would be better to always return two slices instead of an Option. The second slice could just be empty if the buffer did not wrap.
That way implementing buf_mut() and as_static_buf for the second slice would be easier.

[Sympatron]

I refactored it so that it does not need the Option anymore. This actually made the implementation and use of the SplitGrantR simpler.

Also added some basic sanity checks.

[jamesmunns]

Overall looks good, thanks for the PR! I made some comments, let me know what you think.

[Sympatron]

Implemented all the changed you supposed. Especially the borrow checker one was a good catch, Didn't think about that.

[jamesmunns]

Looks great, thanks!