stream: fix reading LibuvStream into array

[vtjnash]

Adds a new abstraction take!(dst::Array{T,N}, src::Array{T,N}) for doing an efficient copyto! equivalent. Previously it was assumed that compact did this automatically, which wasn't a great assumption.

Fixes #56078

[nhz2]

This is still broken.

readbytes! is documented to never decrease the size of the input vector, but in this example, it gets changed from 1024 to 10.

function mwe(filepath, nb, max_nb)
    write(filepath, ones(UInt8, nb))
    p = open(`cat $filepath`; read=true)
    b = Vector{UInt8}(undef, 1024)
    nr = readbytes!(p, b, max_nb)
    return nr, b
end
nr, b = mwe(tempname(), 10, 10000)

(10, UInt8[0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01])