If layout allows provide a slice view into primitive list

[mmacedoeu]

As I could see to get a memory aligned [u64] in capnproto you must use a primitive list with T = u64/UInt64 but them it is unclear how to get access to a slice of it

[dwrensha]

Correct, there is currently no way to get a slice into the data, for a couple of reasons:

1. Endianness. The data will always be encoded in little-endian order, but the library needs to also work on big-endian systems. I suppose maybe we could conditionally omit the hypothetical to_slice() function on big-endian systems?
2. List upgrades (discussed here). Primitive lists are permitted to be upgraded to struct lists, so primitive_list::Reader needs to be able to deal with cases where the elements are not tightly packed. I suppose a hypothetical to_slice() function could return an error if the elements are tightly packed?

[dwrensha]

See related discussion on #97.

[dwrensha]

capnp::raw::get_list_bytes() was added in 6683d7a