Move newline handling down into parser.

This lets us make it a grammar feature and not a lexer trick.
Newlines are explicitly ignored in some parts of the grammar. This
lets, for example, things like "|" ignore the newline when used as
an operator, but not when used as a parameter delimiter in a block
argument.

test/for/newline_after_for.wren

@@ -0,0 +1,2 @@
+for // expect error
+(i in [1, 2, 3]) IO.print(i)

test/for/newline_before_in.wren

@@ -0,0 +1,2 @@
+for (i // expect error
+  in [1]) IO.print(i)

test/for/syntax.wren

@@ -1,20 +1,14 @@
 // Single-expression body.
-for (i in [1, 2, 3]) IO.print(i)
+for (i in [1]) IO.print(i)
 // expect: 1
-// expect: 2
-// expect: 3
 
 // Block body.
-for (i in [1, 2, 3]) {
+for (i in [1]) {
   IO.print(i)
 }
 // expect: 1
-// expect: 2
-// expect: 3
 
-// Newline after "for".
-for
-(i in [1, 2, 3]) IO.print(i)
-// expect: 1
-// expect: 2
-// expect: 3
+// Newline after "in".
+for (i in
+  [1]) IO.print(i)
+// expect: 1