Switch to keybase go-crypto (for some elliptic curve key) + test (#1925)

* Switch to keybase go-crypto (for some elliptic curve key) + test

* Use assert.NoError 

and add a little more context to failing test description

* Use assert.(No)Error everywhere 🌈

and assert.Error in place of .Nil/.NotNil

vendor/github.com/keybase/go-crypto/curve25519/cswap_amd64.s

@@ -0,0 +1,88 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This code was translated into a form compatible with 6a from the public
+// domain sources in SUPERCOP: http://bench.cr.yp.to/supercop.html
+
+// +build amd64,!gccgo,!appengine
+
+// func cswap(inout *[5]uint64, v uint64)
+TEXT ·cswap(SB),7,$0
+	MOVQ inout+0(FP),DI
+	MOVQ v+8(FP),SI
+
+	CMPQ SI,$1
+	MOVQ 0(DI),SI
+	MOVQ 80(DI),DX
+	MOVQ 8(DI),CX
+	MOVQ 88(DI),R8
+	MOVQ SI,R9
+	CMOVQEQ DX,SI
+	CMOVQEQ R9,DX
+	MOVQ CX,R9
+	CMOVQEQ R8,CX
+	CMOVQEQ R9,R8
+	MOVQ SI,0(DI)
+	MOVQ DX,80(DI)
+	MOVQ CX,8(DI)
+	MOVQ R8,88(DI)
+	MOVQ 16(DI),SI
+	MOVQ 96(DI),DX
+	MOVQ 24(DI),CX
+	MOVQ 104(DI),R8
+	MOVQ SI,R9
+	CMOVQEQ DX,SI
+	CMOVQEQ R9,DX
+	MOVQ CX,R9
+	CMOVQEQ R8,CX
+	CMOVQEQ R9,R8
+	MOVQ SI,16(DI)
+	MOVQ DX,96(DI)
+	MOVQ CX,24(DI)
+	MOVQ R8,104(DI)
+	MOVQ 32(DI),SI
+	MOVQ 112(DI),DX
+	MOVQ 40(DI),CX
+	MOVQ 120(DI),R8
+	MOVQ SI,R9
+	CMOVQEQ DX,SI
+	CMOVQEQ R9,DX
+	MOVQ CX,R9
+	CMOVQEQ R8,CX
+	CMOVQEQ R9,R8
+	MOVQ SI,32(DI)
+	MOVQ DX,112(DI)
+	MOVQ CX,40(DI)
+	MOVQ R8,120(DI)
+	MOVQ 48(DI),SI
+	MOVQ 128(DI),DX
+	MOVQ 56(DI),CX
+	MOVQ 136(DI),R8
+	MOVQ SI,R9
+	CMOVQEQ DX,SI
+	CMOVQEQ R9,DX
+	MOVQ CX,R9
+	CMOVQEQ R8,CX
+	CMOVQEQ R9,R8
+	MOVQ SI,48(DI)
+	MOVQ DX,128(DI)
+	MOVQ CX,56(DI)
+	MOVQ R8,136(DI)
+	MOVQ 64(DI),SI
+	MOVQ 144(DI),DX
+	MOVQ 72(DI),CX
+	MOVQ 152(DI),R8
+	MOVQ SI,R9
+	CMOVQEQ DX,SI
+	CMOVQEQ R9,DX
+	MOVQ CX,R9
+	CMOVQEQ R8,CX
+	CMOVQEQ R9,R8
+	MOVQ SI,64(DI)
+	MOVQ DX,144(DI)
+	MOVQ CX,72(DI)
+	MOVQ R8,152(DI)
+	MOVQ DI,AX
+	MOVQ SI,DX
+	RET