1. std.array.replace cannot receive null

It should run following code:
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int[] a = [1, 4, 5];
replace(a, 1u, 2u, null);
assert(a == [1, 5]);
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2. std.array.replace have strange behavior when array.length shrinks.

It should run following code:
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int[] a = [1, 4, 5];
replace(a, 1u, 4u, [2]);
assert(a == [1, 2]);
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