Adds conditional byteswapping intrinsics

[auroranockert]

They simply byte-swap an integer to a specific endian, like the hton* functions in C.

These intrinsics are synthesized, so maybe they should be in another file. But since they are just a single line of code each, based on the bswap intrinsics and aren't really intended for public consumption I thought they would fit in the intrinsics file.

The next step working on this could be to expose a trait / generic function for byteswapping.

[brson]

Is there an issue associated with this? I'm not clear on the motivation here. Also, I would prefer they be put somewhere else since they aren't true compiler intrinsics.

[thestinger]

@brson: converting to a common endian is needed for sending binary data over a network or serializing to a portable forward, so it's probably common enough

freebsd/linux include these in endian.h and POSIX has ones for network byte order (big endian) in arpa/inet.h

[emberian]

We already have code that does this too. It's in io, it's unconditional and can't take advantage of the llvm intrinsic. +1 for this.

[auroranockert]

@cmr (or someone else) Do you happen to know what would be the best API for this if we want to be able to optimize the implementation in IO?

trait ByteSwappable {
fn to_little_endian() -> T;
fn to_big_endian() -> T;
fn byte_swap() -> T
}

where T is a [u8, ..n] or something like that?

[huonw]

@jensnockert -> Self is probably the only thing that works at the moment (there's no way to parameterise the n in [u8, .. n] until associated constants work).

[emberian]

@jensnockert well io could use the functions in this PR as it is (and get rid of read_{be,le}_int_n and just use .read in the read_le_i16 etc).

I'm not sure what a good interface for this functionality would be. Might check how other languages handle byte swapping?