Information logger

[t-lohse]

Depends on #165
Fixes #153
This adds a new macro msg that either sends Information through gRPC to the client, or prints it to stdout if Reveaal is run in the CLI.
This implementation works if no parallelism is implemented on a "deeper level", as it uses thread::current.id() to seperate the clients. If this were to change, a new way of handling clients should be implemented. Should I add a todo, or implement a more futureproof version?

msg can be used in a number of ways:

msg!("message") | msg!("{}", message) // makes severity = Info, and subject = "General"
msg!("Subject", msg: "message") | msg!("Subject", msg: "{}", message) // makes severity = Info
msg!(severity, subject: "Subject", msg: "message") | msg!("Subject", msg: "{}", message) // Sets everything explicitly

I am unsure if it should be so explicit, we could reduce them to look like a function call? (msg!(severity, subject, message))