Range-ify std.path.filenameCmp()

[WalterBright]

100% unit test coverage

The behavior changes here:

1. decode only if case insensitive or the arguments are different string encodings
2. if decoding, treat invalid UTF encodings as if they were replacementDchar's, rather than throwing an exception

This has the effect of making the function:

1. faster
2. nothrow and @nogc

Being a low level function, of course this affects everything up the call stack.

[quickfur]

Wouldn't this cause odd behaviour if given two strings with invalid encodings in the same positions, thus causing unequal strings to compare equal?

[WalterBright]

There are 3 options with invalid encodings:

1. throw exception
2. terminate program with some sort of error or assert
3. replace with replacementDchar

Each has advantages and disadvantages. There is no correct solution other than validating your data beforehand. Consider also that floating point NaNs are regarded as equivalent even though they can have different bit patters.

I tend to favor the one with the lowest runtime cost, which is (3). It is far more practical for the programmer to add extra checks if he wants (1) or (2) than it is if he wants to remove them. In fact, to remove them he'd have to reimplement these routines.

[andralex]

these two popFronts should go in the for imho

[andralex]

lgtm modulo the unnecessary assert(0)

[andralex]

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