Finalize run-end-encoding support

[paleolimbot]

After #507 we have REE support! There are a few places this still needs to be integrated:

• Integration tests
• R bindings
• Python bindings
• Potentially a helper for iterating along it or resolving an element out of it (e.g., using a binary search). This might also be combined with a helper for dictionary arrays to help consumers support a wider variety of the new encodings

Also, we should review the C++ implementation for more utilities/gotchas/cases we need to ensure validate (or fail to validate) as per #507 (comment)

https://github.com/apache/arrow/blob/main/cpp/src/arrow/util/ree_util.h
and
https://github.com/apache/arrow/blob/main/cpp/src/arrow/util/ree_util.cc
are good sources to look at.