Convert XxHash64 to VarHandle

[dain]

Replace sun.misc.Unsafe with VarHandle for memory access in XxHash64.

Changes

• Replace Unsafe.getLong/getInt/getByte with VarHandle.get()
• Replace Unsafe.copyMemory with System.arraycopy
• Add public hash(byte[]) API for direct byte array hashing
• Inline static hash implementation to avoid method call overhead

Performance

Inlining the updateBody method in the implementation was necessary to avoid an 18% regression at 64 bytes with VarHandle. After inlining, performance is equivalent for the byte[] API. The Slice API has a slight regression
at 64 bytes (~1ns per invocation) which I suspect is due to interaction between VarHandle bounds checking and Slice wrapper methods.

Size	unsafe_bytes	unsafe_slice	varhandle_bytes	varhandle_slice
8B	1.4 ns	1.6 ns	1.9 ns	1.6 ns
64B	6.5 ns	6.8 ns	6.5 ns	7.8 ns
1KB	65 ns	65 ns	63 ns	64 ns
64KB	3,968 ns	4,065 ns	4,032 ns	4,065 ns