add evidence contract for reference

plugin/dapp/wasm/contracts/evidence/evidence.cpp

+#include "../common.h"
+#include "evidence.hpp"
+#include <string.h>
+
+int set(int64_t n) {
+	size_t l = getENVSize(n);
+	//暂不支持动态数组 char* value = new char[l]; 给定静态数组长度表示字符串长度上限
+	char value[1024] = {0};
+	getENV(n, value, l);
+	printlog(value, l);
+	return 0;
+}
\ No newline at end of file

plugin/dapp/wasm/contracts/evidence/evidence.hpp

+#ifdef __cplusplus //而这一部分就是告诉编译器，如果定义了__cplusplus(即如果是cpp文件，
+extern "C" { //因为cpp文件默认定义了该宏),则采用C语言方式进行编译
+#endif
+
+int set(int64_t n);
+
+#ifdef __cplusplus
+}
+#endif
\ No newline at end of file