Append a newline without blocking multiple times when printing to STDOUT/STDERR

For #160 (closed).

A small test script:

import std::stdio::stdout
import std::stdio::stderr

let mut x = stdout.print('Hello, world!')
stdout.print(x)
x = stdout.print('Bye, world!')
stdout.print(x)
x = stdout.print()
stdout.print(x)
x = stdout.print("---")
stdout.print(x)

x = stderr.print('Hello, world!')
stderr.print(x)
x = stderr.print('Bye, world!')
stderr.print(x)
x = stderr.print()
stderr.print(x)
x = stderr.print("---")
stderr.print(x)

When piping stdout/err to separate files the output in each should be:

Hello, world!
14
Bye, world!
12

1
---
4