Prevent all() from overwriting its arguments

Currently, a validator can be a callable that raises an exception. Since there is no explicit requirement for it to return the value it was passed, it is possible for a validator that doesn't return anything when it successfully validates a value to cause a failure when used in conjunction with some other validator via the `all()` function. See the following example:

```
import voluptuous as v

def isodd(x):
    if (x % 2.0) == 0.0:
        raise ValueError

val1 = v.Schema(isodd)
val2 = v.Schema(v.range(0.0, 10.0))
val3 = v.Schema(v.all(isodd, v.range(0.0, 10.0)))

x = 3.0

# None of these should raise an exception:
val1(x)
val2(x)
val3(x)
```

The pull request prevents this problem from occurring by not recycling the variable used to pass the value being tested to each of the validators passed to the `all()` function.