convert(::FD{T,N}, ::Int128) and convert(::FD{T,N}, ::UInt128) allocate BigInts

[Drvi]

For example

julia> using FixedPointDecimals

julia> @time convert(FixedDecimal{Int,2}, Int128(1))
  0.000005 seconds (7 allocations: 128 bytes)
FixedDecimal{Int64,2}(1.00)

allocates a BigInt because a widemul is used to carry out the multiplication between the coefficient and the Int128 value during the conversion.
This is problematic performance-wise.

One workaround would be to convert the coefficient to an Int128 and then use Base.mul_with_overflow, throwing an InexactError manually on overflow.

[NHDaly]

Fixed in #84.