Allow for multiple backends

[Erk-]

Currently we only generate a single backend for a given futhark file, but there is nothing that should make it impossible to generate multiple. At the moment it will not work since we use features but that will be resolved by #31.

So to bring this change to frutition we need to have different names for each backend and then be able to select which to link and use at compile time in the generated project.

[Erk-]

Hi @SafariMonkey, I can see that you have done this on your own fork. Would you be willing to try and upstream it with the new Enum style selection implemented in #31?