Optimize StatementContextBase layout

We currently store CopyHistory as a reference to an object, costing us 4b/8b of storage in StatementContextBase.

StatementContextBase is the bread&butter of memory/CPU trade-offs and it does not have a single bit of storage left.

At the end of the day, CopyHistory encodes the following information:

• last CopyType
• CopyableNode.isAugmenting()
• AddedByUsesAware.isAddedByUses()

This currently boils down to 2+log2(4)=4 significant bits, for which we are storing 32/64 bits.

Refactor StatementContextBase.copyHistory to be

private final byte copyHistory;

This frees up 3-7 bytes of storage, which provides 24bits of additional free storage.

Another candidate is StatementContextBase.completedPhase, which again is 4/8 bytes of storage, but it only stores 7 states, i.e. none + ModelProcessingPhase which there are 6 of.

We could use only 3 bits for storing that state, but that would incur masking operations for sub-byte access. Since this is an utterly hot path, let's instead keep

private byte completedPhase;

Which again frees up 3-7 bytes of storage.

Combining these two, though, we get real gains – we trim down 8-16 bytes of storage for everything 64bit, while also a creating an unused 2-byte alignment gap. There also remain 4 unused bits in copyHistory, giving us comfortable 20bits if we need to store more information.