Rayon no longer required, new bytes attribute, and more!

• Rayon is no longer required as dependency. Thanks to @oligamiq!
• The attribute type BYTES was added (new OpenEXR feature!)
• Finally support PXR24 on big-endian architecture! (It was a fundamental problem on big endian architectures).
• Never panic on i32/i64 conversion overflow, instead we always return an error now.