Disallow new with any non-map argument

[JoeStrout]

Right now you can do new "foo" or new 42 or new null, and you get back a map with __isa set to the argument. But this is a weird map that does not have access to the standard map methods; and doing isa tests on this produces inconsistent results between the C# and C++ implementations. The resulting beast is just plain weird (and even weirder if you used a funcRef or NaN as your argument!), and it's difficult to decide how it ought to behave, or why you would ever have a valid use for it.

So, let's just throw a runtime error if you use new with any argument that is not a map. If you manually assign to __isa, well, we probably can't stop you (without impacting all map-entry assignments) — but we should explicitly document that the resulting behavior is undefined.

[JoeStrout]

...While we're at it, think about what we should do if some confused user does new string, new list, or new funcRef. Unless they're just poking the edges of the language on purpose, this is almost certainly an error.

[juh9870]

Would be good to throw informative error, akin to

• new string - \ If you want to construct a new string, use a string literal syntax "like this"
• new array - \ If you want to construct a new array, use an array syntax ["like", "this"] etc