release notes: Note new two-step allocator
release notes: Note new two-step allocator

(cherry picked from commit 5a58634ec6f79af175685d97f4051cb5532d4b22)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc