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/open_block.wren

@@ -0,0 +1,16 @@
+import "io" for File
+
+var stash
+File.open("test/io/file/open_block.wren") {|file|
+  System.print(file is File) // expect: true
+  System.print(file.isOpen) // expect: true
+  stash = file
+}
+
+// Closes after block.
+System.print(stash.isOpen) // expect: false
+
+// Returns null.
+System.print(File.open("test/io/file/open_block.wren") {|file|}) // expect: null
+
+// TODO: Test a fiber aborting inside the block.