Nail down Unknown vs Known function calls

[doyougnu]

We have a decent overview of this now, but we really should have a much more robust section (probably in the STG chapter #14) that covers:

• known vs unknown functions: across different boundaries: modules, packages
• fast and slow stg applies, genutils
• how to check for known and unknown (prof report)
• performance consequences of exporting a function from a module
• How these interact with interface files
• probably something about unfoldings

Some resources:

• https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/rts/haskell-execution/function-calls
• https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/rts/haskell-execution/calling-convention
• the eval-apply paper. This is where a lot of the theory gets covered for the STG maching (besides SPJ's book) but we need to know the implementation to really understand the performance consequences.

Basically, as a user of GHC I want to:

• understand the consequences of exporting a function with respect to runtime performance.

[doyougnu]

the generic apply functions are referenced in the upcoming perf chapter because they crop up during a conversation about PAPs and info tables. Be sure to refine the link for this ticket.

• update the perf chapter