Logging

[colin-kiegel]

I suggest we implement some basic logging - this seems to be a popular crate and enables clients of our library to implement the actual logging backend:
https://crates.io/crates/log/

Possible use cases:

• It would be useful to log some INFO or WARNING if variables of templates are not set for example.
  • 'INFO: rendering template "X"'
  • 'WARN: template variable "X" uninitialized'
• We could use it for debugging
  • 'DEBUG: parsed token "X" at ..'

I think we could even make this optional in our cargo [features] section. If someone compiles without that feature, we can fallback to real noop-macros. To optimize the runtime costs for different usages, we could even have two compile flags "log" < "debug", where debug implies log, but not otherwise. Our tests could then benefit of some debug logging.

Example from their documentation:

#[macro_use]
extern crate log;

pub fn shave_the_yak(yak: &Yak) {
    info!(target: "yak_events", "Commencing yak shaving for {:?}", yak);

    loop {
        match find_a_razor() {
            Ok(razor) => {
                info!("Razor located: {}", razor);
                yak.shave(razor);
                break;
            }
            Err(err) => {
                warn!("Unable to locate a razor: {}, retrying", err);
            }
        }
    }
}

I don't know if we really need LogLevel::Error, because I think we should just return a Result::Err in these cases - so I don't expect us to use this log level, but anyway, here is the list from the log-crate:

pub enum LogLevel {
    Error, // log OR debug
    Warn, // log OR debug
    Info,   // log OR debug
    Debug, // debug
    Trace,  // debug
}

What do you think?