Fix compile errors for interpolated AnyVals

[jcracknell]

The interpolation destructuring added in 3.6.0 attempts to convert interpolated messages to slf4j message formatting calls. These have signature (String, Object*)Unit, and as such any interpolations involving value types result in compilation errors. The simplest fix I came up with for this is to invoke toString on such interpolations, as they cannot be null.

The implementation also doesn't escape {} appearing in string parts, and doesn't account for Scala's wonky escape handling for interpolators.

[lightbend-cla-validator]

Hi @jcracknell,

Thank you for your contribution! We really value the time you've taken to put this together.

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Please review the new CLA and sign it before we proceed with reviewing this pull request:

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[analytically]

Thanks!!