Add integer conversions in `JSONValue.get`

`JSONValue.get` now allows to convert a stored `uinteger` or `integer` into any
signed or unsigned integer. The conversion is performed with `std.conv.to`, and
throws a `ConvException` in case of an integer overflow;

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auto json = parseJSON(`{"a": 123}`);
writeln(json["a"].get!ubyte);
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