Optimize copy_in a bit
Optimize copy_in a bit

io::copy has to first copy onto the stack and then into the writer, but
we can just explicitly call read_to_end which knows it's dealing with a
Vec.

ddrmanxbxfr/rust-postgres,sfackler/rust-postgres,achanda/rust-postgres,sfackler/rust-postgres,ddrmanxbxfr/rust-postgres,achanda/rust-postgres,tempbottle/rust-postgres,tempbottle/rust-postgres