Ability to defer functions, closes #403

[odino]

Sometimes it is very helpful to guarantee a certain function is executed
regardless of what code path we take: you can use the defer keyword for
this.

echo(1)
defer echo(3)
echo(2)

When you schedule a function to be deferred, it will executed right at
the end of the current scope. A defer inside a function will then
execute at the end of that function itself:

echo(1)
f fn() {
    defer echo(3)
    echo(2)
}
fn()
echo(4)

You can defer any callable: a function call, a method or even a system
command. This can be very helpful if you need to run a cleanup function
right before wrapping up with your code:

defer `rm my-file.txt`
"some text" > "my-file.txt"

...
...
"some other text" >> "my-file.txt"

In this case, you will be guaranteed to execute the command that removes
my-file.txt before the program closes.

Be aware that code that is deferred does not have access to the return value
of its scope, and will supress errors -- if a defer block messes up you're
not going to see any error. This behavior is experimental, but we would most
likely like to give this kind of control through try...catch...finally.