Changes

Summary

1. merged
2. DiagnosticSeverity according to implementation https://github.com/Microsoft/vscode-languageserver-node instead of documentation https://github.com/Microsoft/language-server-protocol/blob/master/protocol.md;
3. support for syslog messages; clarified shutdown;
4. prefer stable state -- reduce repeated diagnostics;
5. more uniform pending_input / pending_output; explicit Document_Model.uri; tuned;
6. unused;
7. precise full_range and thus proper try_restrict in Snapshot.cumulate;
8. clarified protocol errors;
9. clarified signature;
10. unused;
11. clarified options;
12. clarified signature: maintan Text.Length within Line.Document;
13. clarified modules;
14. clarified signature: explicit Length to avoid implicit mistakes;
15. tuned;
16. more uniform treatment of input/output wrt. client; support for diagnistic messages; misc tuning;
17. tuned;
18. more uniform treatment of "bad" like other messages (with serial number);
19. clarified modules;
20. support for diagnostics;