Fix Travis to really test 32 bits

To make sure 32 bits arch is being tested, try the following test which should
fail in 32 bits mode:
```go
package utils

import (
	"math/bits"
	"testing"
)

func TestBreak(t *testing.T) {
	if bits.UintSize != 64 {
		panic("bits.UintSize != 64")
	}
}
```

.travis.yml

@@ -4,5 +4,22 @@ language: go
 go:
   - "1.12"
 
+# Travis overrides the GOARCH env var probably in its `travis_setup_go`
+# function, so we need a work around...
+jobs:
+  include:
+    - name: "Unit Tests 64 bit arch"
+      env:
+        - XGOARCH="amd64"
+    - name: "Unit Test 32 bit arch"
+      env:
+        - XGOARCH="386"
+
 env:
   - GO111MODULE=on
+
+before_install:
+  - export GOARCH=$XGOARCH
+
+script:
+  - go test -v ./...

ff/element.go

@@ -513,15 +513,33 @@ func (z *Element) String() string {
 
 // ToBigInt returns z as a big.Int in Montgomery form
 func (z *Element) ToBigInt(res *big.Int) *big.Int {
+	if bits.UintSize == 64 {
 		bits := (*[4]big.Word)(unsafe.Pointer(z))
 		return res.SetBits(bits[:])
+	} else {
+		var bits [8]big.Word
+		for i := 0; i < len(z); i++ {
+			bits[i*2] = big.Word(z[i])
+			bits[i*2+1] = big.Word(z[i] >> 32)
+		}
+		return res.SetBits(bits[:])
+	}
 }
 
 // ToBigIntRegular returns z as a big.Int in regular form
 func (z Element) ToBigIntRegular(res *big.Int) *big.Int {
 	z.FromMont()
+	if bits.UintSize == 64 {
 		bits := (*[4]big.Word)(unsafe.Pointer(&z))
 		return res.SetBits(bits[:])
+	} else {
+		var bits [8]big.Word
+		for i := 0; i < len(z); i++ {
+			bits[i*2] = big.Word(z[i])
+			bits[i*2+1] = big.Word(z[i] >> 32)
+		}
+		return res.SetBits(bits[:])
+	}
 }
 
 // SetBigInt sets z to v (regular form) and returns z in Montgomery form
@@ -548,9 +566,19 @@ func (z *Element) SetBigInt(v *big.Int) *Element {
 	}
 	// v should
 	vBits := vv.Bits()
+	if bits.UintSize == 64 {
 		for i := 0; i < len(vBits); i++ {
 			z[i] = uint64(vBits[i])
 		}
+	} else {
+		for i := 0; i < len(vBits); i++ {
+			if i%2 == 0 {
+				z[i/2] = uint64(vBits[i])
+			} else {
+				z[i/2] |= uint64(vBits[i]) << 32
+			}
+		}
+	}
 	return z.ToMont()
 }
 

utils/showarchbits_test.go

@@ -0,0 +1,11 @@
+package utils
+
+import (
+	"fmt"
+	"math/bits"
+	"testing"
+)
+
+func TestShowArchBits(t *testing.T) {
+	fmt.Printf("Architecture is %v bits\n", bits.UintSize)
+}