Consider exporting LogStr

[MichaelXavier]

I frequently work with libraries that expose their logging API via LogStr, e.g. persistent. I would like to route these log messages to a different logging system but LogStr does not export sufficient constructors to do so. I was wondering if you'd be open to creating an Internal module tree and exporting basically what you've got in System.Log.FastLogger.LogStr right now. I'd even say you could just rename that module. This would of course carry the caveat that the API can break any time under the Internal module.

As it stands right now, for every log message I will have to show it to convert it from a builder to a string, then convert that back into the same exact Builder type for my log sink.

[kazu-yamamoto]

Yes. Internal makes sense to me. Would you send a PR?