WIP: Error doc

I'm working on syscall/src/error.rs. I don't expect this to be merged, because it would break a lot of code. It's more a list of proposals than a merge request or an invitation to discuss.

First of all, I added some comments and tests. Just run cargo test to see the test results.

In the first commit, I also added some boundary checks on Error::new and Error::mux. The main idea is that one should not be allowed to create an error without using a predefined constant, or trying to mux a huge Ok value. This must trigger a panic and be fixed in the kernel. I also removed the public access to STR_ERROR. I think that a module should expose services rather than data. The only use of STR_ARRAY I could find is: "given an errno, what is the error description?". Why not give directly this service?

Second commit: I was rather confused by an Error::demux returning Ok. I expected it to always return an Error. I created a result module for Result, mux and demux. When I read result::demux, it seems clear to me that I will get a Result. That may seem a detail, but if the code is easy to read, you avoid bugs.

Obviously, this would break a lot of libraries!

What do you think about this?