Unify the fat pointer layouts in back::abi

[nrc]

There are three versions of these layouts for functions, traits, and slices, but the compiler relies on these being the same. We should replace them with a single layout: fat_ptr_data and fat_ptr_extra or something. While we're at it, we should probably find uses of len/length names in the DST code and replace it with something more generic.

[ruuda]

I merged the constants. Could you elaborate on the length names? Are there more constants like these?

[apasel422]

What still needs to be done here, if anything?

[brson]

Looks done.