marshal.store generates invalid JSON

[FedericoCeratto]

Non-printable string characters should be escaped, e.g. \u0000
This example leaves a \0 byte in the generated JSON

type Foo = object of RootObj
  version: string

let m = Foo(version: "\0")
let f = newFileStream("foo.json", fmWrite)
f.store(m)
f.close()