EulerIntrinsicKey

A key naming EulerIntrinsic specifically, distinct from
any string a real leaf can print as (a leaf's printed form never starts with a NUL) --
so it shares nothing with the ordinary named constant e, which prints the same
text EulerIntrinsic does. Both denote the same number and
both are kept out of this key's collision space on purpose:
EulerIntrinsic is a distinguished reference a binder over
the name e must not capture, and merging the two into one e-class -- which
Key(AngouriMath.Entity) keying on Entity.Stringize() alone would do -- silently
discards that distinction on the next extraction. Caught in code review before this PR
was merged.