Update to tengo v2 (#976)

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+<p align="center">
+  <img src="https://raw.githubusercontent.com/d5/tengolang-share/master/logo_400.png" width="200" height="200">
+</p>
+
+# The Tengo Language
+
+[![GoDoc](https://godoc.org/github.com/d5/tengo?status.svg)](https://godoc.org/github.com/d5/tengo)
+[![Go Report Card](https://goreportcard.com/badge/github.com/d5/tengo)](https://goreportcard.com/report/github.com/d5/tengo)
+[![CircleCI](https://circleci.com/gh/d5/tengo.svg?style=svg)](https://circleci.com/gh/d5/tengo)
+[![Sourcegraph](https://sourcegraph.com/github.com/d5/tengo/-/badge.svg)](https://sourcegraph.com/github.com/d5/tengo?badge)
+
+**Tengo is a small, dynamic, fast, secure script language for Go.** 
+
+Tengo is **[fast](#benchmark)** and secure because it's compiled/executed as
+bytecode on stack-based VM that's written in native Go.
+
+```golang
+/* The Tengo Language */
+fmt := import("fmt")
+
+each := func(seq, fn) {
+    for x in seq { fn(x) }
+}
+
+sum := func(init, seq) {
+    each(seq, func(x) { init += x })
+    return init
+}
+
+fmt.println(sum(0, [1, 2, 3]))   // "6"
+fmt.println(sum("", [1, 2, 3]))  // "123"
+```
+
+> Test this Tengo code in the
+> [Tengo Playground](https://tengolang.com/?s=0c8d5d0d88f2795a7093d7f35ae12c3afa17bea3)
+
+## Features
+
+- Simple and highly readable
+  [Syntax](https://github.com/d5/tengo/blob/master/docs/tutorial.md)
+  - Dynamic typing with type coercion
+  - Higher-order functions and closures
+  - Immutable values
+- [Securely Embeddable](https://github.com/d5/tengo/blob/master/docs/interoperability.md)
+  and [Extensible](https://github.com/d5/tengo/blob/master/docs/objects.md)
+- Compiler/runtime written in native Go _(no external deps or cgo)_
+- Executable as a
+  [standalone](https://github.com/d5/tengo/blob/master/docs/tengo-cli.md)
+  language / REPL
+- Use cases: rules engine, [state machine](https://github.com/d5/go-fsm),
+  data pipeline, [transpiler](https://github.com/d5/tengo2lua)
+
+## Benchmark
+
+| | fib(35) | fibt(35) |  Type  |
+| :--- |    ---: |     ---: |  :---: |
+| Go | `48ms` | `3ms` | Go (native) |
+| [**Tengo**](https://github.com/d5/tengo) | `2,349ms` | `5ms` | VM on Go |
+| Lua | `1,416ms` | `3ms` | Lua (native) |
+| [go-lua](https://github.com/Shopify/go-lua) | `4,402ms` | `5ms` | Lua VM on Go |
+| [GopherLua](https://github.com/yuin/gopher-lua) | `4,023ms` | `5ms` | Lua VM on Go |
+| Python | `2,588ms` | `26ms` | Python (native) |
+| [starlark-go](https://github.com/google/starlark-go) | `11,126ms` | `6ms` | Python-like Interpreter on Go |
+| [gpython](https://github.com/go-python/gpython) | `15,035ms` | `4ms` | Python Interpreter on Go |
+| [goja](https://github.com/dop251/goja) | `5,089ms` | `5ms` | JS VM on Go |
+| [otto](https://github.com/robertkrimen/otto) | `68,377ms` | `11ms` | JS Interpreter on Go |
+| [Anko](https://github.com/mattn/anko) | `92,579ms` | `18ms` | Interpreter on Go |
+
+_* [fib(35)](https://github.com/d5/tengobench/blob/master/code/fib.tengo):
+Fibonacci(35)_  
+_* [fibt(35)](https://github.com/d5/tengobench/blob/master/code/fibtc.tengo):
+[tail-call](https://en.wikipedia.org/wiki/Tail_call) version of Fibonacci(35)_  
+_* **Go** does not read the source code from file, while all other cases do_  
+_* See [here](https://github.com/d5/tengobench) for commands/codes used_
+
+## Quick Start
+
+A simple Go example code that compiles/runs Tengo script code with some input/output values:
+
+```golang
+package main
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/d5/tengo/v2"
+)
+
+func main() {
+	// Tengo script code
+	src := `
+each := func(seq, fn) {
+    for x in seq { fn(x) }
+}
+
+sum := 0
+mul := 1
+each([a, b, c, d], func(x) {
+	sum += x
+	mul *= x
+})`
+
+	// create a new Script instance
+	script := tengo.NewScript([]byte(src))
+
+	// set values
+	_ = script.Add("a", 1)
+	_ = script.Add("b", 9)
+	_ = script.Add("c", 8)
+	_ = script.Add("d", 4)
+
+	// run the script
+	compiled, err := script.RunContext(context.Background())
+	if err != nil {
+		panic(err)
+	}
+
+	// retrieve values
+	sum := compiled.Get("sum")
+	mul := compiled.Get("mul")
+	fmt.Println(sum, mul) // "22 288"
+}
+```
+
+## References
+
+- [Language Syntax](https://github.com/d5/tengo/blob/master/docs/tutorial.md)
+- [Object Types](https://github.com/d5/tengo/blob/master/docs/objects.md)
+- [Runtime Types](https://github.com/d5/tengo/blob/master/docs/runtime-types.md)
+  and [Operators](https://github.com/d5/tengo/blob/master/docs/operators.md)
+- [Builtin Functions](https://github.com/d5/tengo/blob/master/docs/builtins.md)
+- [Interoperability](https://github.com/d5/tengo/blob/master/docs/interoperability.md)
+- [Tengo CLI](https://github.com/d5/tengo/blob/master/docs/tengo-cli.md)
+- [Standard Library](https://github.com/d5/tengo/blob/master/docs/stdlib.md)