Fix parsing of hierarchical identifiers

the lexor didn't allow numbers in the IDENTIFIER name,
and the parser wasn't very nice with IDENTIFIER errors.

Signed-off-by: Stephen Williams <steve@icarus.com>

vpi/sdf_lexor.lex

@@ -65,7 +65,7 @@ static int yywrap(void)
       return REAL_NUMBER;
 }
 
-[a-zA-Z]+ {
+[a-zA-Z][a-zA-Z0-9]* {
       return lookup_keyword(yytext);
 }
 

vpi/sdf_parse.y

@@ -198,6 +198,10 @@ cell_instance
       { $$ = strdup(""); }
   | '(' K_INSTANCE '*' ')'
       { $$ = 0; }
+  | '(' K_INSTANCE error ')'
+      { vpi_printf("%s:%d:SDF ERROR: Invalid/malformed INSTANCE argument\n",
+		   sdf_parse_path, @2.first_line);
+	    $$ = strdup(""); }
   ;
 
 timing_spec_list