[mysten-util-mem] Align fastcrypto versions with Sui

[jonas-lj]

The mysten-util-mem dependency in Sui currently points to a revision on the bump_fastcrypto branch to ensure that the fastcrypto versions (which are also git revision) in Sui and mysten-util-mem are the same. This should be fixed, eg. by pointing to a version of fastcrypto instead of revisions. This should be done once the interfaces of fastcrypto are more stable.