rx::global<T> is very large

[graphitemaster]

consider shrinking the global_node size, can use several tricks here:

• tag bits
• xor linked-list (for grouped and ungrouped links)
• bump point allocator on top of static storage for storage of both the global and the arguments (pick something like 16 KiB, that should be enough for anyone)

[graphitemaster]

Did the XOR linked-list approach and tag bits stuff, still unsure if a global allocator for globals is a good idea, it would trim 64 bytes off each global.

[graphitemaster]

Made even smaller

[graphitemaster]

We can use the tag bits for storing the flags inside m_argument_store as the last space saving optimization here.

[graphitemaster]

This is as small as it's ever going to be.