This updates the implementation to reflect a part of the D ABI spec
that has been removed.  There should never be a bare integer value
encoded into a template argument list.  Integers are always prefixed
by `i' if they are positive or `N' if they are negative.

Have verified this indeed is the case using the code below, and
updated all coverage tests to match the compiler.

    module demangle;

    template test(alias T)
    {
        void test() { auto t = T; }
    }

    pragma(msg, (test!(cast(byte)123)).mangleof);
    pragma(msg, (test!(cast(int)123)).mangleof);
    pragma(msg, (test!(cast(short)123)).mangleof);
    pragma(msg, (test!(cast(ubyte)123)).mangleof);
    pragma(msg, (test!(cast(uint)123)).mangleof);
    pragma(msg, (test!(cast(ushort)123)).mangleof);
    pragma(msg, (test!(cast(long)123)).mangleof);
    pragma(msg, (test!(cast(ulong)123)).mangleof);
    pragma(msg, (test!(true)).mangleof);
    pragma(msg, (test!(false)).mangleof);
    pragma(msg, (test!('\x0a')).mangleof);
    pragma(msg, (test!(' ')).mangleof);
    pragma(msg, (test!('A')).mangleof);
    pragma(msg, (test!('~')).mangleof);
    pragma(msg, (test!('\u03e8')).mangleof);
    pragma(msg, (test!('\U000186a0')).mangleof);

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