Add a --release flag

[xermicus]

This came up in #927, I'll open an issue so I won't forget this.

The idea is to have a --release flag for compilation, similar to what cargo does. This would allow us to enable certain optimization only for release builds (e.g. omitting debug prints in release builds).

[seanyoung]

This sounds useful, but right now there is nothing to differentiate between so called release and non-release builds. Before adding such a flag, we first need a feature which we want to switch on or off release.

When get to that point, we can have another discussion about what such an option should be called. --release might not be the most descriptive name, we'd have to decide when we get there.

[xermicus]

Closed by #1227