Distinguish between ternary's : and arrow fn's return type (#573)

* Distinguish between ternary's : and arrow fn's return type

* Correctly parse nested arrow functions inside conditional expressions

Defer the conversion of arrow function parameters to assignable nodes so that
it is possible to use the (invalid) ast to get the exact position of the (wrong)
arrow functions.

* Check params of arrow fns w/ type params or w/o return type

* Fix also async functions

* Add test from prettier

https://github.com/prettier/prettier/issues/2194

* Don't check arrow params if they are valid at the first attemp

* Use state instead of relying on the "noArrowParamsConversion" parameter

* Remove noArrowParamsConversion

test/fixtures/flow/regression/issue-58-ambiguous/actual.js

@@ -0,0 +1,4 @@
+// This can be parsed in two ways:
+// a ? b : (c => ((d): e => f))
+// a ? ((b): c => d) : (e => f)
+a ? (b) : c => (d) : e => f;

test/fixtures/flow/regression/issue-58-ambiguous/options.json

@@ -0,0 +1,3 @@
+{
+    "throws": "Ambiguous expression: wrap the arrow functions in parentheses to disambiguate. (4:4)"
+}