Fix undefined behavior in deserialize_header()

[jesterhodl]

This attempts to fix #30 by:

• consolidating read_u32_le(), read_u64_le(), write_u32_le(), write_u64_le()
• renames them to deserialize/serialize to avoid conflicts with protocol/serialize.c functions with the same name, but different type and signature
• removes UBSan bug from deserialize_header() by using deserialize_u32_le()