Use ordered maps

[fenollp]

Fixes #645

[bitomaxsp]

What kind of help is needed here? We (@northvolt) are super eager to see that released :)

[fenollp]

Hey @bitomaxsp I'd love some help! Right now I'm stuck solving this:

FAIL | Issue301 (0.00s)
     | panic: runtime error: invalid memory address or nil pointer dereference [recovered]
     | 	panic: runtime error: invalid memory address or nil pointer dereference
     | [signal SIGSEGV: segmentation violation code=0x1 addr=0x58 pc=0x805850]
     | goroutine 19 [running]:
     | testing.tRunner.func1.2({0x888260, 0xc6abe0})
     | 	/snap/go/10008/src/testing/testing.go:1396 +0x24e
     | testing.tRunner.func1()
     | 	/snap/go/10008/src/testing/testing.go:1399 +0x39f
     | panic({0x888260, 0xc6abe0})
     | 	/snap/go/10008/src/runtime/panic.go:884 +0x212
     | github.com/getkin/kin-openapi/openapi3.TestIssue301(0x0?)
     | 	/home/pete/wefwefwef/kin-openapi.git/openapi3/issue301_test.go:32 +0x250
     | testing.tRunner(0xc0001ac820, 0x947b40)
     | 	/snap/go/10008/src/testing/testing.go:1446 +0x10b
     | created by testing.(*T).Run
     | 	/snap/go/10008/src/testing/testing.go:1493 +0x35f
FAIL | 	github.com/getkin/kin-openapi/openapi3	0.005s

From there I believe the design satisfies me. The rest of the work should be straightforward(TM)!

Could you describe your needs WRT this patch a bit?