Prevent Ed25519 signature malleability

Prevent Ed25519 signature malleability

Add a check that ensures that the upper half s of an Ed25519 signature is
bounded by the group order, i.e, 0 <= s < order. This is required by the
Verify procedure in RFC 8032, section 5.1.7, step 1, and prevents simple
modifications of signatures such as adding (a multiple of) the group order
to the upper half of the signature.

Found with EdDSA testcase 63 of project Wycheproof.

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