specialize signbit(::FixedDecimal)

[lcontento]

The default implementation of signbit is quite slow for BigInt and Int128. MWE:

using FixedPointDecimals
using BenchmarkTools

signbit2(x) = signbit(x.i);

x = FixedDecimal{BigInt, 10}(1);
@btime signbit($x);
@btime signbit2($x);

x = FixedDecimal{Int128, 10}(1);
@btime signbit($x);
@btime signbit2($x);