Update maps EEP to use match instead of equal

We have updated notes for the => operator to make it clear
it will *match* keys. We have also updated the functions
in the maps module to mention *match* semantics (for example,
`maps:from_list/1`).

We have also removed examples that use `lists:key*` functions
since those use equal. Notes regarding ordering have not
been affected by this commit.
Mention maps operations happen in arbitrary order

Note the `maps:foldl/3` and `maps:foldr/3` were removed in favor
of `maps:fold/3`, since iteration happens in arbitrary order.
Note maps:size/1 happens in constant time

We have removed the comparisons with `length(maps:to_list(Map))`
to avoid confusion regarding the time-complexity of the size
operation.