Bug 21557 – std.encoding and std.utf do not use replacement char consistently

std.utf uses std.utf.replacementDchar, which seems is what unicode actually expects to happen. However, std.encoding uses std.encoding.INVALID_SEQUENCE , defined here: https://github.com/dlang/phobos/blob/master/std/encoding.d#L481 This later value isn't the one expected by the unicode standard. Additionally, the later value creates a problem because now there are two values to handle for replacement chars.

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