Comparison against BoringSSL-generated test vectors showed mismatches
with the pure Go implementation of curve25519. The problem was narrowed
down to a missing mask in feFromBytes(). This diff adds the mask,
bringing this back in line with the reference implementation and
RFC 7748:

    When receiving such an array, implementations of X25519 (but not
    X448) MUST mask the most significant bit in the final byte.  This is
    done to preserve compatibility with point formats that reserve the
    sign bit for use in other protocols and to increase resistance to
    implementation fingerprinting.

Fixes golang/go#30095

Change-Id: If7efc0e2acd6efb761d6e3cb89cec359d7d81cb1
Reviewed-on: https://go-review.googlesource.com/c/161257
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>