structLookupCache

// The structLookupCache caches StructOf lookups.
// StructOf does not share the common lookupCache since we need to pin
// the memory associated with *structTypeFixedN.

addToCache := func(t Type) Type {
var ts []Type
if ti, ok := structLookupCache.m.Load(hash); ok {
ts = ti.([]Type)
}
structLookupCache.m.Store(hash, append(ts, t))
return t
}

如果通过map来获取 struct field的话，会有个问题，map是无序的，而struct 的filed顺序不一致时不同的类型，hash值不一样，比较相等也如此

因此需要对key进行排序，否则会内存泄漏