compiler: allow to use init function

Process `init()` functions after global variables has been processed.
It's body is saved into the `_initialize` method where all
initialization is performed.

pkg/compiler/init_test.go

@@ -0,0 +1,62 @@
+package compiler_test
+
+import (
+	"math/big"
+	"testing"
+)
+
+func TestInit(t *testing.T) {
+	t.Run("Simple", func(t *testing.T) {
+		src := `package foo
+		var a int
+		func init() {
+			a = 42
+		}
+		func Main() int {
+			return a
+		}`
+		eval(t, src, big.NewInt(42))
+	})
+	t.Run("Multi", func(t *testing.T) {
+		src := `package foo
+		var m = map[int]int{}
+		var a = 2
+		func init() {
+			m[1] = 11
+		}
+		func init() {
+			a = 1
+			m[3] = 30
+		}
+		func Main() int {
+			return m[1] + m[3] + a
+		}`
+		eval(t, src, big.NewInt(42))
+	})
+	t.Run("WithCall", func(t *testing.T) {
+		src := `package foo
+		var m = map[int]int{}
+		func init() {
+			initMap(m)
+		}
+		func initMap(m map[int]int) {
+			m[11] = 42
+		}
+		func Main() int {
+			return m[11]
+		}`
+		eval(t, src, big.NewInt(42))
+	})
+	t.Run("InvalidSignature", func(t *testing.T) {
+		src := `package foo
+		type Foo int
+		var a int
+		func (f Foo) init() {
+			a = 2
+		}
+		func Main() int {
+			return a
+		}`
+		eval(t, src, big.NewInt(0))
+	})
+}