partially make to!string nothrow

Since dup is now correctly inferred, same-width string conversion is now nothrow. So I marked the unittests as such.

I also removed a useless enforce, which was covered as an assert in a precondition. So now, integral to string conversion is also nothrow this means `text("there are ", 5, "bunnies")` is now entirely nothrow.

Other types of conversion use `format`, so that's still throwing. I think I can workaround that in a future pull though.

Also, removed all those useless debug statements.

So yeah, pretty trivial.