[ISSUE #68]Change from pub fn to pub const fn for from_static_str method (#69)

Author: mxsm

src/cheetah_string.rs

@@ -227,7 +227,7 @@ impl CheetahString {
     }
 
     #[inline]
-    pub fn from_static_str(s: &'static str) -> Self {
+    pub const fn from_static_str(s: &'static str) -> Self {
         CheetahString {
             inner: InnerString::StaticStr(s),
         }