refactor!: move macro generated types to pseudo-private module

[austinabell]

Noticed these as well, should have never been exported. Assuming they aren't used anywhere.

These changes are a bit opinionated so input would be appreciated.

1. Metadata moved because the format isn't stable and will likely change
2. BorshIntoStorageKey moved because it's just used by macros and can be very confusing with BorshStorageKey

[ChaoticTempest]

Makes sense to me to have this change as long as its only being used in the macros

[itegulov]

Makes sense to me. Metadata API is most likely going to change in the next release so hiding in this release is definitely a good idea