Break up `Settings` into smaller structs

As far as I can tell, the fields within `Settings` aren't *intrinsicly*
related. They just happen to be initialized the same way (in particular
prior to the rest of `DynFlags`), and that is why they are grouped
together.

Within `Settings`, however, there are groups of settings that clearly do
share something in common, regardless of how they anything is
initialized.

In the spirit of GHC being a library, where the end cosumer may choose
to initialize this configuration in arbitrary ways, I made some new data
types for thoses groups internal to `Settings`, and used them to define
`Settings` instead. Hopefully this is a baby step towards a general
decoupling of the stateful and stateless parts of GHC.