Resolve Elixir 1.20 type checker warnings

[tomciopp]

Two sources, no behavior change:

The non guard expansions of is_decimal/1 are a case with a %Decimal{}
clause and a catch all. When the argument is statically known to be a
%Decimal{} the type checker proves the catch all unreachable and warns
at every such call site, including the doctest. Marking the quoted
expansion generated: true declares the catch all deliberate,
following the convention for macro generated code.

Three tests pass intentionally invalid input to assert the raise
(new/1 with an atom, to_integer/1 with a NaN coefficient, round/1 with
nil). Routing those calls through apply/3 keeps the argument opaque to
the type checker, which does not check dynamic dispatch, while
exercising exactly the same failure path.

[ericmj]

Thank you! 💜