Fixes for -preview=rvaluerefparam

[dkorpel]

See dlang/dmd#17068 (comment)

hasToString is a bit of a tangle, it tries to force passing FormatSpec by const ref for customPutWriter, but not for constCharSinkFormatSpec. It's not necessary to require it, I guess copying it is discouraged because it is slower than needed, but it's hard to statically enforce this in the presence of overloads.

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