optimization: Don't include ASCII characters in Unicode tables

The ASCII subset of Unicode is fixed and will never change, so we don't
need to generate tables for it with every new Unicode version. This
saves a few bytes of static data and speeds up `char::is_control` and
`char::is_grapheme_extended` on ASCII inputs.

Since the table lookup functions exported from the `unicode` module will
give nonsensical errors on ASCII input (and in fact will panic in debug
mode), I had to add some private wrapper methods to `char` which check
for ASCII-ness first.

Author: Kmeakin

alloc/src/str.rs

@@ -418,9 +418,8 @@ impl str {
         }
 
         fn case_ignorable_then_cased<I: Iterator<Item = char>>(iter: I) -> bool {
-            use core::unicode::{Case_Ignorable, Cased};
-            match iter.skip_while(|&c| Case_Ignorable(c)).next() {
-                Some(c) => Cased(c),
+            match iter.skip_while(|&c| c.is_case_ignorable()).next() {
+                Some(c) => c.is_cased(),
                 None => false,
             }
         }

core/src/char/methods.rs

@@ -969,7 +969,43 @@ impl char {
     #[must_use]
     #[inline]
     pub(crate) fn is_grapheme_extended(self) -> bool {
-        unicode::Grapheme_Extend(self)
+        !self.is_ascii() && unicode::Grapheme_Extend(self)
+    }
+
+    /// Returns `true` if this `char` has the `Cased` property.
+    ///
+    /// `Cased` is described in Chapter 4 (Character Properties) of the [Unicode Standard] and
+    /// specified in the [Unicode Character Database][ucd] [`DerivedCoreProperties.txt`].
+    ///
+    /// [Unicode Standard]: https://www.unicode.org/versions/latest/
+    /// [ucd]: https://www.unicode.org/reports/tr44/
+    /// [`DerivedCoreProperties.txt`]: https://www.unicode.org/Public/UCD/latest/ucd/DerivedCoreProperties.txt
+    #[must_use]
+    #[inline]
+    #[doc(hidden)]
+    #[unstable(feature = "char_internals", reason = "exposed only for libstd", issue = "none")]
+    pub fn is_cased(self) -> bool {
+        if self.is_ascii() { self.is_ascii_alphabetic() } else { unicode::Cased(self) }
+    }
+
+    /// Returns `true` if this `char` has the `Case_Ignorable` property.
+    ///
+    /// `Case_Ignorable` is described in Chapter 4 (Character Properties) of the [Unicode Standard] and
+    /// specified in the [Unicode Character Database][ucd] [`DerivedCoreProperties.txt`].
+    ///
+    /// [Unicode Standard]: https://www.unicode.org/versions/latest/
+    /// [ucd]: https://www.unicode.org/reports/tr44/
+    /// [`DerivedCoreProperties.txt`]: https://www.unicode.org/Public/UCD/latest/ucd/DerivedCoreProperties.txt
+    #[must_use]
+    #[inline]
+    #[doc(hidden)]
+    #[unstable(feature = "char_internals", reason = "exposed only for libstd", issue = "none")]
+    pub fn is_case_ignorable(self) -> bool {
+        if self.is_ascii() {
+            matches!(self, '\'' | '.' | ':' | '^' | '`')
+        } else {
+            unicode::Case_Ignorable(self)
+        }
     }
 
     /// Returns `true` if this `char` has one of the general categories for numbers.