Fixes global numeric types to have max of signed encoding (#442)

This adjusts towards the exiting code which used int32/64 instead of
uint32/64. The reason is that the spec indicates intepretation as signed
numbers, which affects the maximum value.

See https://www.w3.org/TR/wasm-core-1/#value-types%E2%91%A2

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/u64/u64_test.go

@@ -0,0 +1,39 @@
+package u64
+
+import (
+	"encoding/binary"
+	"math"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestBytes(t *testing.T) {
+	tests := []struct {
+		name  string
+		input uint64
+	}{
+		{
+			name:  "zero",
+			input: 0,
+		},
+		{
+			name:  "half",
+			input: math.MaxUint32,
+		},
+		{
+			name:  "max",
+			input: math.MaxUint64,
+		},
+	}
+
+	for _, tt := range tests {
+		tc := tt
+
+		t.Run(tc.name, func(t *testing.T) {
+			expected := make([]byte, 8)
+			binary.LittleEndian.PutUint64(expected, tc.input)
+			require.Equal(t, expected, LeBytes(tc.input))
+		})
+	}
+}