Now that nightly supports portable_simd, we can attempt to replace the SIMD implementations with portable_simd.
In the current state, this does not avoid unsafe entirely, because while the BlockDescriptor struct is just a new-type for u16, Rust does not support a safe transmute to a u16 slice, which we need to load the data into SIMD registers.

This can be solved by changing the BlockDescriptor struct into a type alias. Alternatively, we could change block descriptors from u16 to [u16; 16] to force alignment, which would make transmuting entirely obsolete, since we can just operate on the entire superblock.

Since those decisions are part of #9, where we want to change the backing structs, and force alignment, this PR is blocked until those implementation choices are made.