accept '=>' anywhere '->' is acccepcted
add tests for es6-style arrow function literal
do not require single parameter of arrow function should be closed with parentheses
add tests for arrow functions
add arrow function-like type notation
add tests for arrow-function-like type notation
Revert "add tests for arrow-function-like type notation"

This reverts commit ccca4793c933eaab454df6237a33a671831ef049.
Revert "add arrow function-like type notation"

This reverts commit 89277a784e34311f5f306689adb9c2ba8730ffd7.
update completion test
add a failing test regarding #247