[#1] Create an example contract

Contract has PutNumber and GetNumber methods

Signed-off-by: Ekaterina Lebedeva <ekaterina.lebedeva@yadro.com>

impulse/config.yml

@@ -0,0 +1,3 @@
+name: "Example"
+safemethods: []
+supportedstandards: []

impulse/contract.go

@@ -0,0 +1,35 @@
+package contract
+
+import (
+	"github.com/nspcc-dev/neo-go/pkg/interop/storage"
+)
+
+// Prefixes used for contract data storage.
+const (
+	// prefixNumber contains map from the key to stored number.
+	prefixNumber byte = 0x00
+)
+
+const (
+	keySize = 5
+)
+
+func GetNumber(key []byte) int {
+	if len(key) != keySize {
+		panic("Invalid key size")
+	}
+	ctx := storage.GetContext()
+	num := storage.Get(ctx, append([]byte{prefixNumber}, key...))
+	if num == nil {
+		panic("Cannot get number")
+	}
+	return num.(int)
+}
+
+func PutNumber(key []byte, num int) {
+	if len(key) != keySize {
+		panic("Invalid key size")
+	}
+	ctx := storage.GetContext()
+	storage.Put(ctx, append([]byte{prefixNumber}, key...), num)
+}