Consider to migrate the expression engine

[suzuki-shunsuke]

Currenly, we use antonmedv/expr as the expression engine. antonmedv/expr is very useful, but there are some problems.

• It isn't supported to define variables
• It isn't supported to nest the loop

So we consider to migrate the other library.

Embeddable Scripting Languages

• https://github.com/robertkrimen/otto - not versioning
• https://github.com/d5/tengo

[suzuki-shunsuke]

We decided to migrate to d5/tengo

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