RbxWriteInterleaved Trait

[krakow10]

The intention here is to improve the code from #545, and provide a counterpart to the RbxReadInterleaved trait defined in #530. This removes the "foreign" impl of functions in core.rs onto ChunkBuilder (defining functions onto a struct from another module), and organizes the write_interleaved_* functions into a trait. The initialize_bytes_with function was a cross-module hack, so has been inlined into the write_interleaved_bytes impl. The write test in core_read_write.rs is returned to its former glory because the trait is defined for Vec<u8>, and the constants are deduplicated.

Note:

• The -> io::Result<()> result is actually infallible (barring allocation failure panics), but is there to match the surrounding function signatures. I have another commit 5968ec4 if we want to make it infallible, since it only ever operates on a Vec.