Add Port::fequal for longdouble comparisons.

This makes me a little nervous inside to change, as it would be useful to have access to ```realsize``` and ```realpad``` from port for the sake of portability.

Maybe should change real comparisons to:
``` return ::abs(x, y) <= MAX(LDBL_EPSILON, LDBL_EPSILON * MAX(::abs(x), ::abs(y)));```

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