Add `endian.h`

Relates to #173

Adds the following functions from endian.h, as described here:

• uint16_t be16toh(uint16_t);
• uint32_t be32toh(uint32_t);
• uint64_t be64toh(uint64_t);
• uint16_t htobe16(uint16_t);
• uint32_t htobe32(uint32_t);
• uint64_t htobe64(uint64_t);
• uint16_t htole16(uint16_t);
• uint32_t htole32(uint32_t);
• uint64_t htole64(uint64_t);
• uint16_t le16toh(uint16_t);
• uint32_t le32toh(uint32_t);
• uint64_t le64toh(uint64_t);