avoid toLower in std.algorithm.sort compare alias

[Jay Norwood]

While playing with sorting the unzip archive entries I tried use 
of the last example in 
http://dlang.org/phobos/std_algorithm.html#sort

std.algorithm.sort!("toLower(a.name) < 
toLower(b.name)",std.algorithm.SwapStrategy.stable)(entries);

It was terribly slow  for sorting the 34k entries in my test 
case.  I'd guess it is doing the toLower call on both strings for 
every compare.

It was much, much faster to add an entries.lowerCaseName = 
std.string.toLower(entries.name) foreach entry prior to the sort 
execution and then use

std.algorithm.sort!("a.lowerCaseName < b.lowerCaseName 
",std.algorithm.SwapStrategy.stable)(entries);

The difference was on the order of 10 secs vs no noticeable delay 
when executing the sort operation in the debugger.