Overhaul LZH code

• Rename variables and macros to be more readable.
• Split encoding and decoding variables into separate structs so they don't all need to be on the stack
• Stop using signed values for anything This also stops depending on left-shifting into the sign bit "working" as expected... something compilers can optimize out
• Use size-based types instead of short/long
• Make the encoded buffer endian-agnostic (convert to little-endian)
• Clean up lzh_update() a bit
• Enable long-disabled overflow checks
• Accept a size_t buffer size argument to encode/decode