Fixed Str parser so it handles apostrophes inside words.

Previously the --smart option would trigger exponential slowdown
in text heavy with intraword apostrophes.  This fixes the problem.

Str is now a bunch of NormalChars followed by a list of StrChunks.
A StrChunk is either a bunch of NormalChars, an underscore followed
by alphanumeric, or (in smart mode) an apostrophe.

markdown_parser.leg

@@ -404,8 +404,15 @@ Space = Spacechar+
         { $$ = mk_str(" ");
           $$->key = SPACE; }
 
-Str = < NormalChar (NormalChar | '_'+ &Alphanumeric)* >
-        { $$ = mk_str(yytext); }
+Str = a:StartList < NormalChar+ > { a = cons(mk_str(yytext), a); }
+      ( StrChunk { a = cons($$, a); } )*
+      { if (a->next == NULL) { $$ = a; } else { $$ = mk_list(LIST, a); } }
+
+StrChunk = < (NormalChar | '_'+ &Alphanumeric)+ > { $$ = mk_str(yytext); } |
+           AposChunk
+
+AposChunk = &{ extension(EXT_SMART) } '\'' &Alphanumeric
+      { $$ = mk_element(APOSTROPHE); }
 
 EscapedChar =   '\\' !Newline < [-\\`|*_{}[\]()#+.!><] >
                 { $$ = mk_str(yytext); }
@@ -664,7 +671,7 @@ EnDash = '-' &Digit
 EmDash = ("---" | "--")
          { $$ = mk_element(EMDASH); }
 
-SingleQuoteStart = '\'' ![)!\],.;:-? \t\n] !( ( "s" | "t" | "m" | "ve" | "ll" | "re" ) !Alphanumeric )
+SingleQuoteStart = '\'' ![)!\],.;:-? \t\n]
 
 SingleQuoteEnd = '\'' !Alphanumeric