add encode and test for double, boolean, int, long and null

[jasondeng1997]

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[bookjiang]

pls assign to me

[jasondeng1997]

assign to bookjiang

[VaderKai]

pls assign to me

[jasondeng1997]

assign to VaderKai

[felix021]

The new plan is to implement fastcodec for Hessian2, more details: #28