libsyntax: accept only whitespace with the PATTERN_WHITE_SPACE property

This aligns with unicode recommendations and should be stable for all future
unicode releases. See http://unicode.org/reports/tr31/#R3.

This renames `libsyntax::lexer::is_whitespace` to `is_pattern_whitespace`
so potentially breaks users of libsyntax.

src/test/run-pass/parser-unicode-whitespace.rs

@@ -9,10 +9,14 @@
 // except according to those terms.
 
 
-// Beware editing: it has numerous whitespace characters which are important
+// Beware editing: it has numerous whitespace characters which are important.
+// It contains one ranges from the 'PATTERN_WHITE_SPACE' property outlined in
+// http://unicode.org/Public/UNIDATA/PropList.txt
+//
+// The characters in the first expression of the assertion can be generated
+// from: "4\u{0C}+\n\t\r7\t*\u{20}2\u{85}/\u{200E}3\u{200F}*\u{2028}2\u{2029}"
 pub fn main() {
-    assert_eq!(4 + 　7 * 2
+assert_eq!(4+
 
-
-/ 3 * 2 , 4 + 7 * 2 / 3 * 2);
+7   * 2…/‎3‏*
2
, 4 + 7 * 2 / 3 * 2);
 }