fix typecons printing system & clean imports

Depends on #2729 
Hmm, you seem to be right. I get the same error with the following code:

    void main()
    {
        //Error: undefined identifier std
        std.stdio.writeln("test");
    }

But as soon as I import, say, std.conv, the program compiles. I don't know why std.conv would import std.stdio, so it must be the buggy `import module: symbol` style imports at work. My bad.