Improve set matching

[evhub]

Should match dictionary mapping semantics, such that

{1, 2, 3} = some_set

matches if {1, 2, 3} <= some_set, rather than {1, 2, 3} == some_set, with explicit syntax in the form of

{1, 2, 3, *()} = some_set

for checking {1, 2, 3} == some_set.

Note that this would be a breaking change, as was the previous change to dict matching.

[evhub]

Should also add support for set letter matches, with f{} only matching frozensets and m{} only matching collections.Counters with support for properly matching repeated elements.