Bug 1026344 - Part 1: Don't use memcpy to copy style structs. r=dbaron

This is in preparation for nsStyleCoords becoming non-POD types.  The
use of memcpy is a bit of a trap when modifying the style structs, and
it likely has marginal value now that anything with an nsStyleCoord
can't be memcpy-initialized.
Bug 1026344 - Part 2: Make nsStyleUnit a uint8_t-sized enum so we can use it on nsStyle{Sides,Corners}. r=dbaron
Bug 1026344 - Part 3: Rename CalcValue to PixelCalcValue. r=dbaron
Bug 1026344 - Part 4: Make nsStyleCoord::Calc refcounted and introduce a non-refcounted nsStyleCoord::CalcValue. r=dbaron

This means that style structs holding computed calc() values are now able to be
stored in the rule tree.
Bug 1026345 - Remove the now unused nsStyleContext allocation functions. r=dbaron
Bug 1026903 - Make nsStyleContext members private. r=dbaron