Add SafeParse() for value types

[hanssens]

Requires no further introduction; the TryParse function is quite cumbersome, due to the out ref:

public int GetValue(string numberAsText) {
    int value;
    int.TryParse(numberAsText, out value);
    return value;
}

In C# 7 we might get rid of this. But until then - and for backwards compatibility - this is still cumbersome and a SafeParse() for these value types would be a clean solution.