什么是 Golang Comparable Types

Golang 中有很多时候要用到comparable types，例如比较struct的时候，例如map里的 key。弄清楚有哪些类型是comparable的非常重要。

https://golang.org/ref/spec#Comparison_operators

Boolean values
Integer values
Floating point values
Complex values
String values
Pointer values
Channel values
Interface values
Struct values are comparable if all their fields are comparable
Array values are comparable if values of the array element type are comparable
A value x of non-interface type X and a value t of interface type T are comparable when values of type X are comparable and X implements T