Split traits for plain and bitpacked decoding and fix soundness issue in BitReader::get_batch

[jhorstmann]

Which issue does this PR close?

• Closes Soundness: Unsound alignment contract in public FromBytes trait and BitReader::get_batch #10164.

Rationale for this change

The FromBitpacked trait was introduced in #9662 as a marker for types that could be decoded from bitpacked data, but the implementation still mostly used methods from its FromBytes super-trait. The implementation of BitReader::get_batch that dispatched on the size of the type then required a complex safety contract, which meant that FromBytes had to be an unsafe trait.

Instead of that size dispatch, we can move the unsafe code into a method of the FromBitpacked trait, which makes BitReader::get_batch completely safe. This also removes the need to make any of those traits sealed as proposed in #10166.

What changes are included in this PR?

The responsibilities of the FromBytes and FromBitpacked traits should be much clearer now:

• FromBytes is used for plain encoded data
• FromBitpacked is used for bitpacked data

Unsafe code is moved from BitReader::get_batch into some of the FromBitpacked implementations.

Are these changes tested?

All existing tests pass.

Are there any user-facing changes?

FromBitpacked gains a new method and is no longer a sub-trait of FromBytes, so this might be breaking change. Code that previously implemented FromBytes will need to change to remove the unsafe keyword and BIT_CAPACITY constant.