KDOC 438: Goのバケット構造体はフィールドがないが値を保持している

https://github.com/golang/go/blob/988a20c8c5e2c9eb49f8749e5ee94ce3c964fe59/src/runtime/map_noswiss.go#L149-L160

// A bucket for a Go map.
type bmap struct {
	// tophash generally contains the top byte of the hash value
	// for each key in this bucket. If tophash[0] < minTopHash,
	// tophash[0] is a bucket evacuation state instead.
	tophash [abi.OldMapBucketCount]uint8
	// Followed by bucketCnt keys and then bucketCnt elems.
	// NOTE: packing all the keys together and then all the elems together makes the
	// code a bit more complicated than alternating key/elem/key/elem/... but it allows
	// us to eliminate padding which would be needed for, e.g., map[int64]int8.
	// Followed by an overflow pointer.
}