crypto/bn256: full switchover to cloudflare's code (#16301)

* crypto/bn256: full switchover to cloudflare's code

* crypto/bn256: only use cloudflare for optimized architectures

* crypto/bn256: upstream fallback for non-optimized code

* .travis, build: drop support for Go 1.8 (need type aliases)

* crypto/bn256/cloudflare: enable curve mul lattice optimization

crypto/bn256/cloudflare/gfp_decl.go

@@ -0,0 +1,18 @@
+// +build amd64,!generic arm64,!generic
+
+package bn256
+
+// This file contains forward declarations for the architecture-specific
+// assembly implementations of these functions, provided that they exist.
+
+// go:noescape
+func gfpNeg(c, a *gfP)
+
+//go:noescape
+func gfpAdd(c, a, b *gfP)
+
+//go:noescape
+func gfpSub(c, a, b *gfP)
+
+//go:noescape
+func gfpMul(c, a, b *gfP)