Infer function expressions in matching contexts (#514)

* legalizes omitting types on function expressions within function type contexts
* legalizes omitting any number of arguments

tests/parser/arrow-functions.ts

@@ -4,6 +4,7 @@
 (x?) => x;
 (x?, y?) => x;
 (x?: i32) => x;
+x => x;
 
 // not an array function
 (b ? x : y);