Add emit_binary() to serialize::Encoder trait

[mneumann]

My proposal is to extend trait serialize::Encoder with:

pub trait Encoder {
    fn emit_binary(&mut self, bin: &[u8]);
}

Which special cases emit_seq for u8's. Msgpack for example support both binaries and sequences of values, while sequences of values incur more overhead because it needs to encode the elements type for each element.

Right now it is not possible to efficiently emit binary type, like type bin = ~[u8] using the Encoder trait. With this change it would be possible. Orthogonally a read_binary() should be added to the Decoder trait, but it is not that important, as read_seq can be overloaded accordingly to read both binaries or sequences.

The default behaviour of emit_binary() could be made to emit a sequence, so almost no code need to be changed.

[steveklabnik]

The serialize crate is now out of tree.