This upstreams NatCast and IntCast alone independent of norm_cast in
#3322.
This will allow more efficiently upstreaming parts of Std.Data.Int
relevant for omega.
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Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
@Kha The new `rational.lean` test shows their usefulness. We just
define the monorphic version and a coercion, and get a bunch of `HAdd`
instances for free.