Clean up text handling a bit:

- Rename IO.write -> IO.print.
- Make IO.write not print a newline.
- Support \u Unicode escapes in strings.

test/for/close_over_loop_variable.wren

@@ -1,8 +1,8 @@
 var list = []
 
 for (i in [1, 2, 3]) {
-  list.add(fn IO.write(i))
-  //list.add(fn IO.write(i))
+  list.add(fn IO.print(i))
+  //list.add(fn IO.print(i))
 }
 
 for (f in list) f.call

test/for/closure_in_body.wren

@@ -2,7 +2,7 @@ var list = []
 
 for (i in [1, 2, 3]) {
   var j = i + 1
-  list.add(fn IO.write(j))
+  list.add(fn IO.print(j))
 }
 
 for (f in list) f.call

test/for/for_in_syntax.wren

@@ -1,12 +1,12 @@
 // Single-expression body.
-for (i in [1, 2, 3]) IO.write(i)
+for (i in [1, 2, 3]) IO.print(i)
 // expect: 1
 // expect: 2
 // expect: 3
 
 // Block body.
 for (i in [1, 2, 3]) {
-  IO.write(i)
+  IO.print(i)
 }
 // expect: 1
 // expect: 2
@@ -14,7 +14,7 @@ for (i in [1, 2, 3]) {
 
 // Newline after "for".
 for
-(i in [1, 2, 3]) IO.write(i)
+(i in [1, 2, 3]) IO.print(i)
 // expect: 1
 // expect: 2
 // expect: 3

test/for/only_evaluate_sequence_once.wren

@@ -1,9 +1,9 @@
 var f = fn {
-  IO.write("evaluate sequence")
+  IO.print("evaluate sequence")
   return [1, 2, 3]
 }
 
-for (i in f.call) IO.write(i)
+for (i in f.call) IO.print(i)
 // expect: evaluate sequence
 // expect: 1
 // expect: 2