Implement char.IsLetter, char.IsLetterOrDigit, char.IsSurrogate for non-Unicode characters; fix char.IsWhiteSpace

Since it hasn't been decided if JSIL should include the huge unicode category table ( #225), here's an ANSI-only implementation of char.IsLetter.

Also included is an implementation of IsLetterOrDigit (IsLetter || IsDigit), IsSurrogate (Wikipedia says it's a single range), and a fix for IsWhiteSpace (return statement body was on a separate line, therefore it returned undefined).