Fix block scoping of class declarations

[devongovett]

This fixes an issue where class declarations were not properly block scoped.

const g = 20;

function baz() {
  {
    class g {}
  }

  return g;
}

was compiling to:

const g1 = 20; // <- this is wrong
function baz() {
    {
        class g1 {}
    }
    return g;  // <- this is wrong
}

This was caused by the resolver hoisting class declarations when it shouldn't. Now they are ignored if within a block statement, just like let/const var declarations.