Disable function expressions

[yyc]

Right now, something like this is permitted(!!):

  map(function a(x) {
    return x + 1;
  }, list(1));

It's undocumented and messes up the environment model (no binding for a is created in the calling frame).
Happens because estree is converting it to a function expression (which we include as an undocumented feature) if it sees a function declaration where it expects an expression.

To make js-slang align with the spec, this PR disables both named and unnamed function definition expressions. Only arrow functions are allowed as an expression now.

[coveralls]

Pull Request Test Coverage Report for Build 185

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 60.433%

---

Totals
Change from base Build 178:	0.04%
Covered Lines:	787
Relevant Lines:	1231

---

💛 - Coveralls