Reading files!

- File.close()
- File.open()
- File.read()
- file.readBytes()

And a few other little methods. Still lots more work to do, but it's a
start.

test/io/file/size.wren

@@ -1,7 +1,7 @@
 import "io" for File
 import "scheduler" for Scheduler
 
-System.print(File.size("test/io/file/size.wren")) // expect: 401
+System.print(File.size("test/io/file/size.wren")) // expect: 270
 
 // Runs asynchronously.
 Scheduler.add {
@@ -10,9 +10,4 @@ Scheduler.add {
 
 System.print(File.size("test/io/file/size.wren"))
 // expect: async
-// expect: 401
-
-var error = Fiber.new {
-  System.print(File.size("nonexistent"))
-}.try()
-System.print(error) // expect: no such file or directory
+// expect: 270