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.

src/wren_vm.h

@@ -274,8 +274,7 @@ void* wrenReallocate(WrenVM* vm, void* memory, size_t oldSize, size_t newSize);
 
 // Mark [value] as reachable and still in use. This should only be called
 // during the sweep phase of a garbage collection.
-// TODO: Expose markObj instead?
-void wrenMarkValue(WrenVM* vm, Value value);
+void wrenMarkObj(WrenVM* vm, Obj* obj);
 
 // Sets the current Compiler being run to [compiler].
 void wrenSetCompiler(WrenVM* vm, Compiler* compiler);
@@ -315,9 +314,4 @@ const char* getSymbolName(SymbolTable* symbols, int symbol);
 // Returns the global variable named [name].
 Value findGlobal(WrenVM* vm, const char* name);
 
-// Pushes [function] onto [fiber]'s callstack and invokes it. Expects [numArgs]
-// arguments (including the receiver) to be on the top of the stack already.
-// [function] can be an `ObjFn` or `ObjClosure`.
-void wrenCallFunction(ObjFiber* fiber, Obj* function, int numArgs);
-
 #endif