Nested LLSDArray expressions, e.g.:
LLSD array_of_arrays(LLSDArray(LLSDArray(17)(34))
                              (LLSDArray("x")("y")));
would quietly produce bad results because the outermost LLSDArray was being
constructed with the compiler's implicit LLSDArray(const LLSDArray&) rather
than LLSDArray(const LLSD&) as the reader assumes. Fixed with an explicit copy
constructor to Do The Right Thing.
Generalized LLSDParam<float> specialization into a macro to resolve similar
conversion ambiguities for float, LLUUID, LLDate, LLURI and LLSD::Binary.
Added optional bits= argument to llsd_equals() to permit comparing embedded
Real values using is_approx_equal_fraction() rather than strictly bitwise.
Omitting bits= retains current bitwise-comparison behavior.