Lazy string interpolation in ENSURE

[alexbereznikov]

String interpolation on hot paths leads to performance degradation. We need these messages only when error happened.
With #2453 applied this gives 3x performance boost.

This partially fixes #2451

[ltetak]

Good catch. I guess it would be more standard to move the string interpolation deeper as other libraries do it.
public static void ENSURE(bool conditional, string message, params object[] args)

[alexbereznikov]

@ltetak Sure I thought about that, but profiling says that most of the time was lost in BasePage.ToString() and I'm not quite sure how to avoid calculating it when passing via args

[alexbereznikov]

@ltetak Actually this .ToString() call is redundant as formatter will call it either way. So it should work in the way you proposed, pushed fixed code, please take a look

[ltetak]

@alexbereznikov I am not an owner of this repo. But this looks like a great performance improvement without any risks or side effects.
cc: @mbdavid