Fully implement generic (encode, decode)

[jonasrichard]

We have two possibilities here:

• encode just gets the term and it creates a jsx term. In this case decode should behave accordingly (as right now it does this)
• encode get the term and it generates a new term, and ejson call convert recursively. In this case ejson should decode in that way (decode the content and call post_decode function after that)

The rule can be generic and generic_jsx