remove unused static data for to_lowercase

remove a ton of unused empty strings in char::to_lowercase.
The last column in the lower case table is completely unused.
I noticed this table when analyzing code bloat when compiling to wasm.
using to_lowercase increased the binary size from 50kb to 75kb (which is mostly consisting of empty entries '\u{0}').

Removing the last column is an obvious solution, two others could be done too.
1. The second column has only one entry. Remove that column and create a special handling for that.
2. Group consecutive chars with the same conversion distance.

Author: PSeitz

library/alloc/src/str.rs

@@ -385,16 +385,11 @@ impl str {
                 map_uppercase_sigma(rest, i, &mut s)
             } else {
                 match conversions::to_lower(c) {
-                    [a, '\0', _] => s.push(a),
-                    [a, b, '\0'] => {
+                    [a, '\0'] => s.push(a),
+                    [a, b] => {
                         s.push(a);
                         s.push(b);
                     }
-                    [a, b, c] => {
-                        s.push(a);
-                        s.push(b);
-                        s.push(c);
-                    }
                 }
             }
         }

library/core/src/char/methods.rs

@@ -1018,7 +1018,7 @@ impl char {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn to_lowercase(self) -> ToLowercase {
-        ToLowercase(CaseMappingIter::new(conversions::to_lower(self)))
+        ToLowercase(CaseMappingIter::new_lower(conversions::to_lower(self)))
     }
 
     /// Returns an iterator that yields the uppercase mapping of this `char` as one or more

library/core/src/char/mod.rs

@@ -489,6 +489,13 @@ impl CaseMappingIter {
             CaseMappingIter::Three(chars[0], chars[1], chars[2])
         }
     }
+    fn new_lower(chars: [char; 2]) -> CaseMappingIter {
+        if chars[1] == '\0' {
+            CaseMappingIter::One(chars[0]) // Including if chars[0] == '\0'
+        } else {
+            CaseMappingIter::Two(chars[0], chars[1])
+        }
+    }
 }
 
 impl Iterator for CaseMappingIter {