Use object files for Template Haskell

[mpickering]

Following many of the same reasons as haskell/ghcide#355, it would be beneficial to use object files for supporting TemplateHaskell rather than bytecode because it can be cached between runs.

It is also necessary to stop ghcide caching a typechecked module are all transitive dependencies as this uses too much memory on big code bases, at least until there is a garbage collection story.

This patch is implemented on this branch: https://github.com/mpickering/ghcide/tree/wip/object-files

Questions to resolve:

• How do we decide whether to preemptively generate an object file or not? It is too slow to do on every keystroke.
• Do we want to retain support for ByteCode, perhaps toggling between the two if -fobject-code is passed?

[pepeiborra]

Why not serialize ByteCode?

[mpickering]

@pepeiborra I'm not aware of any way to currently do that, so I would also have to implement it. Do you know that it's possible?

[jneira]

afaiu ghcide master already has it with haskell/ghcide#836! 😃