Fix some errors in huffman.c testing
	modified:   tests/src/test_huffman.c

Moved definition of Symbol out of huffman.c into huffman.h
	modified:   lib/huffman.h
	modified:   src/huffman.c

Added error enums for huffman:
	new file:   lib/huffman_errno.h
Add better testing and error handling of huffman() and unhuffman()

	modified:   src/huffman.c
	modified:   tests/src/test_huffman.c
Explicitly mark private function private by stating name with '_'

	modified:   src/huffman.c
	modified:   tests/src/test_huffman.c
Remove unused #includes

	modified:   src/huffman.c
	modified:   tests/src/test_huffman.c
Refactor to move utility functions out of huffman.c

	modified:   Makefile
	new file:   lib/huffman_util.h
	modified:   src/huffman.c
	new file:   src/huffman_util.c