Make various fixes:

- add feature gate
- add basic tests
- adjust parser to eliminate conflict between `const fn` and associated
constants
- allow `const fn` in traits/trait-impls, but forbid later in type check
- correct some merge conflicts

src/test/compile-fail/check-static-values-constraints.rs

@@ -117,7 +117,7 @@ static mut STATIC14: SafeStruct = SafeStruct {
 //~^ ERROR mutable statics are not allowed to have destructors
     field1: SafeEnum::Variant1,
     field2: SafeEnum::Variant4("str".to_string())
-//~^ ERROR static contains unimplemented expression type
+//~^ ERROR method calls in statics are limited to constant inherent methods
 };
 
 static STATIC15: &'static [Box<MyOwned>] = &[