BorrowDatum, FlatArray, and Text

[workingjubilee]

Introduce a new Rust type that serves as a definition of what Postgres ArrayTypes "really are", called FlatArray. This is meant to be manipulated via &FlatArray<'_, T>... it is an "unsized type". To support this, we also introduce BorrowDatum, which allows borrowing elements directly and is what powers FlatArray's simplicity in iteration, compared to the incredibly jank implementation in pgrx/src/datum/array.rs!

The cost is that it is much more nitpicky about what it can work with, as the type must implement BorrowDatum, which means it must be directly borrowable! This means FlatArray can't be, say, FlatArray<'mcx, String>, and implicitly unbox the elements into an allocated type. You can still map the outputs, of course. The good news is that this means that all FlatArrays must truly be "zero-copy", as the type definition doesn't allow anything else. As part of this compromise, we introduce a new Text type so that it is still possible to work with common array needs like strings.

These are effectively the first varlena types that allow directly interacting with their header instead of "forgetting" it.

[workingjubilee]

This needs more tests, and I'll probably split it into components so that it can be reviewed in smaller chunks, but I'm satisfied with the fundamentals of the implementation here.