Implement `BuildMode` (#298)

* Implement `BuildMode`

* Remove `ignore` since all tests are failing anyway

* Switch to `--release`

* Revert "Switch to `--release`"

This reverts commit 3cb01e10.

* Keep `BuildMode` enum

* Improve readability

* Update changelog

* Make `rustfmt` always report todo's and fixme's

* Remove todo comment

* Fix tests