Make Blob store an Arc<Vec<u8>>

[jdm]

The point of Blob values is that they're immutable buffers, and sometimes they're even slices of immutable buffers, and they're often used as sources of data that will be sent between threads internally. We should support this properly by making the underlying buffer threadsafe, allowing us to pass around references to it rather than cloning it. This will allow us to:

• remove the read_out_buffer method, and rename clone_bytes to get_bytes
• make Blob slices (ie. the result of calling Blob::Slice) share the original buffer, by storing the offset and length of the slice in the new Blob object

Code: components/script/dom/blob.rs

[craftytrickster]

I'd like to take this one.

[jdm]

Go for it!

[asajeffrey]

Should this be Arc<Vec<u8>> or Arc<[u8]>?

[jdm]

I'm not sure how to create the Arc<[u8]> value from the various Blob constructors, so I'm going to go with Arc<Vec<u8>> for the purposes of this issue.