Reduce allocs in AggregateBindable

[smoogipoo]

The benchmarks are below, but only the 2nd result is the significant one to me because it removes a WeakReference alloc. It's still not zero, though, and the other alloc is very hard/impossible to remove with the current structure of things.

Master:

Method	Mean	Error	StdDev	Gen0	Allocated
AddRemoveSource	2.111 us	0.0413 us	0.0776 us	0.2098	1.72 KB
SetValue	20.122 us	0.2580 us	0.2413 us	0.5493	4.69 KB

Without allocating weak references:

Method	Mean	Error	StdDev	Gen0	Allocated
AddRemoveSource	1.760 us	0.0352 us	0.0481 us	0.2041	1.67 KB
SetValue	18.815 us	0.3394 us	0.2834 us	0.5493	4.69 KB

Struct:

Method	Mean	Error	StdDev	Gen0	Allocated
AddRemoveSource	1.580 us	0.0169 us	0.0141 us	0.1411	1.16 KB
SetValue	18.746 us	0.0980 us	0.0917 us	0.5493	4.69 KB

[peppy]

👍 👍