Start adding support for runtime errors:

- Primitives can now signal an error.
- An error prints a callstack and ends the current (and right 
  now only) fiber.
- Arithmetic operators error if the operand is not a number.
- Real error for method not found.
- Associate source file name, and method names with functions.
- Debug line number info for functions.

Unfortunately, this regresses perf on fib about 15%, mostly
because of the better checking in arithmetic ops.

There's still a bunch of clean up to do, but this is a big step
in the right direction.

test/method/not_found.wren

@@ -0,0 +1,3 @@
+class Foo {}
+
+(new Foo).someUnknownMethod // expect runtime error: Receiver does not implement method 'someUnknownMethod'.