Start getting module loading working.

Right now, it uses a weird "import_" method on String which
should either be replaced or at least hidden behind some syntax.

But it does roughly the right thing. Still lots of corner cases to
clean up and stuff to fix. In particular:

- Need to handle compilation errors in imported modules.
- Need to implicitly import all core and IO types into imported module.
- Need to handle circular imports.
  (Just need to give entry module the right name for this to work.)

test/module/multiple_variables/module.wren

@@ -0,0 +1,8 @@
+// nontest
+var Module1 = "from module one"
+var Module2 = "from module two"
+var Module3 = "from module three"
+var Module4 = "from module four"
+var Module5 = "from module five"
+
+IO.print("ran module")

test/module/multiple_variables/multiple_variables.wren

@@ -0,0 +1,14 @@
+var Module1 = "module.wren".import_("Module1")
+var Module2 = "module.wren".import_("Module2")
+var Module3 = "module.wren".import_("Module3")
+var Module4 = "module.wren".import_("Module4")
+var Module5 = "module.wren".import_("Module5")
+
+// Only execute module body once:
+// expect: ran module
+
+IO.print(Module1) // expect: from module one
+IO.print(Module2) // expect: from module two
+IO.print(Module3) // expect: from module three
+IO.print(Module4) // expect: from module four
+IO.print(Module5) // expect: from module five