Better go fmt compatibility, fewer edge cases, faster code

This "major" release provides a revamped implementation which generally increases the compatibility of the redact code with Go's native fmt package.

In particular, it makes it possible to recursively format structs and arrays and expose safe details from them, without including the whole struct or array within redaction markers, with a few caveats. See this commit for details.

Additionally, it introduces new safe types SafeInt, SafeUint and SafeFloat to emit safe numbers in SafePrinter / SafeWriter.