Add support for different backends

[andrevidela]

Codegens for different backends would be nice, currently it does not compile dependencies according to the backend selected for the target binary.

[stefan-hoeck]

I submitted a PR adding a new --cg command-line option. The default codegen to use can also be specified in the pack.toml files.

However, I'm not sure I follow your comment about "not compiling dependencies according to the backend selected". The codegen is only of importance when assembling an executable. Dependencies of such an executable (that is, Idris libraries) will not be "built with a codegen". Or am I missing something.

Anyway, as I mentioned on Discord, the next step will be adding support for building single-file Idris executables.

[stefan-hoeck]

This has been open for a long time, and I'm not sure if this is still an issue or the --gc command-line option is enough. I'm going to close this, because I'd like to clean up the issue tracker a bit. Feel free to reopen, if this requires further work.