Clean up implementation (step 1)

 - Moves all the generators-related modules into a submodule named `generators`.
 - Moves some functions used by generators into the root of this module, because DRY
 - Removes the whole indentation system from the generators (nobody can see the generated code anyway so why would it have indentation?)
 - Adds `/src/gl_generator/target/` to `.gitignore`

(I can't run `cargo test` anymore because of the change in `glfw`, so we'll have to trust travis)