version 1.5.4

Changes since 1.5.3:

• Initial implementation of SIMD acceleration for the XOF (i.e. blake3::Hasher::finalize_xof). This brings long output performance into line with long input performance. Currently AVX-512-only and Unix-only.
• Add build support for "gnullvm" targets (Clang on Windows).
• The "zeroize" feature no longer depends on proc-macros and syn.