Make bindings generation fast and IDE-friendly

[Bromeon]

The current approach of writing the entire bindings into a single file generated.rs seems to be fast regarding compile/link times, but causes problems with the CLion IDE due to the large file size: intellij-rust/intellij-rust#6134

The gdnative-bindings crate comes with two features:

[features]
formatted = []
one_class_one_file = []

The main crate gdnative exposes a feature formatted, which enables both of the above. This seems to fix the CLion issue, but it also looks like it increases compile time significantly.

In my opinion, we should:

• find a good balance between compile time and tool-friendlyness
• make sure the default configuration does not break any tools
• still allow nicely formatted output (for humans), but not by default