compiler: support defer statement

Compile `defer` statement to a TRY/ENDFINALLY opcode pair.

pkg/compiler/defer_test.go

@@ -0,0 +1,82 @@
+package compiler_test
+
+import (
+	"math/big"
+	"testing"
+)
+
+func TestDefer(t *testing.T) {
+	t.Run("Simple", func(t *testing.T) {
+		src := `package main
+		var a int
+		func Main() int {
+			return h() + a
+		}
+		func h() int {
+			defer f()
+			return 1
+		}
+		func f() { a += 2 }`
+		eval(t, src, big.NewInt(3))
+	})
+	t.Run("ValueUnchanged", func(t *testing.T) {
+		src := `package main
+		var a int
+		func Main() int {
+			defer f()
+			a = 3
+			return a
+		}
+		func f() { a += 2 }`
+		eval(t, src, big.NewInt(3))
+	})
+	t.Run("Function", func(t *testing.T) {
+		src := `package main
+		var a int
+		func Main() int {
+			return h() + a
+		}
+		func h() int {
+			defer f()
+			a = 3
+			return g()
+		}
+		func g() int {
+			a++
+			return a
+		}
+		func f() { a += 2 }`
+		eval(t, src, big.NewInt(10))
+	})
+	t.Run("MultipleDefers", func(t *testing.T) {
+		src := `package main
+		var a int
+		func Main() int {
+			return h() + a
+		}
+		func h() int {
+			defer f()
+			defer g()
+			a = 3
+			return a
+		}
+		func g() { a *= 2 }
+		func f() { a += 2 }`
+		eval(t, src, big.NewInt(11))
+	})
+	t.Run("FunctionLiteral", func(t *testing.T) {
+		src := `package main
+		var a int
+		func Main() int {
+			return h() + a
+		}
+		func h() int {
+			defer func() {
+				a = 10
+			}()
+			a = 3
+			return a
+		}`
+		eval(t, src, big.NewInt(13))
+	})
+}