Add streaming read/write

[hoelzro]

Instead of this:

local f = io.open('my-file', 'rb')
local s = f:read '*a'
f:close()

local my_object = pluto.unpersist(perms, s)

allow this:

local f = io.open('my-file', 'rb')
local my_object = pluto.unpersist(perms, s)
f:close()