Mark AsciiString::new() as 'const fn'

Since 'Vec::new()' is already a const function,
there is no difference in generated MIR/ASM whether
'AsciiString::new()' is marked 'const' or not.
I saw issue #56 is waiting to mark this function as 'const',
and so this PR simply marks 'AsciiString::new()' as a const fn.

Thanks for reviewing this PR :)

Author: JOE1994

src/ascii_string.rs

@@ -26,7 +26,7 @@ impl AsciiString {
     /// let mut s = AsciiString::new();
     /// ```
     #[inline]
-    pub fn new() -> Self {
+    pub const fn new() -> Self {
         AsciiString { vec: Vec::new() }
     }