Revert "SI-7469 Remove @deprecated MurmurHash elements"

This reverts commit 7e9c21ffd38711c23f89b5650cc5a32cafe4cd20.

This is still used by SBT; we'll give it a touch longer
to switch to Murmur3.
Unprivatize methods in SyncVar.

These are still used in SBT 0.13.0-M2.

This is a partial revert of ac990c1bc.
Merge pull request #2552 from retronym/topic/restore-murmur

Revert deprecations that break SBT