dialyzer: Modify handling of singleton map key types

The test case loop.erl shows that there is a problem with certain
singleton key types. Here the internal representation toggles between
 #{a | b => ...} and #{a => ..., b => ...}

The choice is to turn #{a | b => ...} into #{a => ..., b => ...} early
(t_from_form()). The aim is to keep as much info as possible (in
pairs). However, including complex singleton keys (tuples, maps) in
this scheme is potentially too costly, and a bit complicated. So one
more choice is made: let atoms and number (and nothing else) be
singleton types, and let complex keys go into the default key.

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