take equals instead of ==

[flaneur2020]

hi Ric,

I tries take advantage of goskiplist as a sorted set in my part time project, but got this error:

panic: runtime error: comparing uncomparable type FileMeta

goroutine 36 [running]:
testing.tRunner.func1(0xc00018e100)
	/usr/local/Cellar/go/1.12/libexec/src/testing/testing.go:830 +0x388
panic(0x12e38a0, 0xc0000a0da0)
	/usr/local/Cellar/go/1.12/libexec/src/runtime/panic.go:522 +0x1b5
github.com/Fleurer/miaomiao/util/goskiplist/skiplist.(*SkipList).Set(0xc0000a2c90, 0x1310a00, 0xc0000b2f40, 0x0, 0x0)
	/Users/fleuria/night/miaomiao/util/goskiplist/skiplist/skiplist.go:416 +0x62a
github.com/fleurer/miaomiao/util/goskiplist/skiplist.(*Set).Add(...)
	/Users/fleuria/night/miaomiao/util/goskiplist/skiplist/skiplist.go:596
github.com/Fleurer/miaomiao/tool.(*Builder).Apply(0xc00019a000, 0xc00004bed8)
	/Users/fleuria/night/miaomiao/tool/builder.go:67 +0x31d
...

FileMeta is a struct so it can not be compared using ==/!=. I've noticed there's a comment about this: Additionally, Ordered instances should behave properly when compared using == and !=. But maybe it's possible to use the lessThan function to get the equality between the values?

Regards.