Performance fix: avoid unnecessary convert on setindex!、copy when type is same. (#38)

* Add `convert(::Type{T}, x::T) where {T<:FD} = x`

This fixes unnecessary copies on `setindex!`, etc.

* Remove "copy constructor" after correct `convert` fix

The "copy constructor" is no longer necessary now that a no-op
`convert` is added, since by default construction falls back to convert.

To verify, after this change, the performance is still correct:
```
julia> f = FixedPointDecimals.FixedDecimal{Int128,2}(3.25)
FixedDecimal{Int128,2}(3.25)

julia> @Btime x = FixedPointDecimals.FixedDecimal{Int128, 2}($f)
  0.036 ns (0 allocations: 0 bytes)
FixedDecimal{Int128,2}(3.25)
```

And `@code_native` shows this to be a simple reinterpret.

Author: NHDaly

src/FixedPointDecimals.jl

@@ -99,9 +99,6 @@ struct FixedDecimal{T <: Integer, f} <: Real
             _throw_storage_error(f, T, n)
         end
     end
-
-    # Copy constructor -- prevents unneeded work to convert between the same types
-    FixedDecimal{T,f}(x::FixedDecimal{T,f}) where {T<:Integer,f} = new{T,f}(x.i)
 end
 
 @noinline function _throw_storage_error(f, T, n)
@@ -274,6 +271,8 @@ function round(::Type{FD{T, f}}, x::Rational, ::RoundingMode{:Nearest}=RoundNear
 end
 
 # conversions and promotions
+convert(::Type{FD{T, f}}, x::FD{T, f}) where {T, f} = x  # Converting an FD to itself is a no-op
+
 function convert(::Type{FD{T, f}}, x::Integer) where {T, f}
     reinterpret(FD{T, f}, T(widemul(x, coefficient(FD{T, f}))))
 end