Bug: parenthesis are not preserved in unary arrow function

Let's say we have an arrow function like this
`const identity = (a) => a;`
after parsing it and then printing ast back, parentheses will be lost:
`const identity = a => a;`.

The logics is described here: https://github.com/benjamn/recast/blob/master/lib/printer.js#L369
We need to check whether parentheses were present in the input and add them in the output if were present.

It is fixed in https://github.com/benjamn/recast/pull/529

Start at lib/printer.js around line 369, where the issue identifies the parenthesis-printing logic, and compare the behavior with the linked pull request 529. Done means parsing and printing a unary arrow function preserves parentheses that were present in the input.

Written by the indexing model from the issue text.