Add config options for spaces around the colon in struct literal fields

In Rust, colons are used for three purposes:

* Type annotations, including type ascription
* Trait bounds
* Struct literal fields

This commit adds options for the last missing of the three purposes,
struct literal fields.

tests/source/configs-space_after_struct_lit_field_colon-false.rs

@@ -0,0 +1,6 @@
+// rustfmt-space_after_struct_lit_field_colon: false
+
+const LOREM: Lorem = Lorem {
+    ipsum:dolor,
+    sit :  amet,
+};

tests/source/configs-space_after_struct_lit_field_colon-true.rs

@@ -0,0 +1,6 @@
+// rustfmt-space_after_struct_lit_field_colon: true
+
+const LOREM: Lorem = Lorem {
+    ipsum:dolor,
+    sit :  amet,
+};

tests/source/configs-space_before_struct_lit_field_colon-false.rs

@@ -0,0 +1,6 @@
+// rustfmt-space_before_struct_lit_field_colon: false
+
+const LOREM: Lorem = Lorem {
+    ipsum:dolor,
+    sit  : amet,
+};

tests/source/configs-space_before_struct_lit_field_colon-true.rs

@@ -0,0 +1,6 @@
+// rustfmt-space_before_struct_lit_field_colon: true
+
+const LOREM: Lorem = Lorem {
+    ipsum:dolor,
+    sit  : amet,
+};

tests/target/configs-space_after_struct_lit_field_colon-false.rs

@@ -0,0 +1,6 @@
+// rustfmt-space_after_struct_lit_field_colon: false
+
+const LOREM: Lorem = Lorem {
+    ipsum:dolor,
+    sit:amet,
+};

tests/target/configs-space_after_struct_lit_field_colon-true.rs

@@ -0,0 +1,6 @@
+// rustfmt-space_after_struct_lit_field_colon: true
+
+const LOREM: Lorem = Lorem {
+    ipsum: dolor,
+    sit: amet,
+};

tests/target/configs-space_before_struct_lit_field_colon-false.rs

@@ -0,0 +1,6 @@
+// rustfmt-space_before_struct_lit_field_colon: false
+
+const LOREM: Lorem = Lorem {
+    ipsum: dolor,
+    sit: amet,
+};

tests/target/configs-space_before_struct_lit_field_colon-true.rs

@@ -0,0 +1,6 @@
+// rustfmt-space_before_struct_lit_field_colon: true
+
+const LOREM: Lorem = Lorem {
+    ipsum : dolor,
+    sit : amet,
+};

tests/target/space-before-type-annotation.rs

@@ -9,5 +9,5 @@ struct S {
     fieldVar : i32,
 }
 fn f() {
-    S { fieldVar : 42 }
+    S { fieldVar: 42 }
 }

tests/target/space-not-after-type-annotation-colon.rs

@@ -10,5 +10,5 @@ struct S {
     fieldVar :i32,
 }
 fn f() {
-    S { fieldVar :42 }
+    S { fieldVar: 42 }
 }