Introduce a new internal type, SnowflakeList, which has better memory
footprint over a regular list or set of roles. It is suspected that
there will be a 9x reduction of memory for every Emoji instance and a
48 byte saving per Member instance. However, these savings will
probably only be evident on larger bots.
As a consequence of this change, Member.roles is now computed lazily.
Currently I am not sure if I want to do the initial sorting on the
SnowflakeList for Member, as this comes with a O(n log n) cost when
creating a Member for little purpose since SnowflakeList.has is not
overly relied on. If CPU time becomes an issue this might change.
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