Fixup BigInt/BigFloat conversions: Restore old implementation, but specialize f=1 case

Before:
```
julia> @Btime convert(FixedDecimal{BigInt,1}, $(BigInt(1)))
  113.644 ns (7 allocations: 128 bytes)
FixedDecimal{BigInt,1}(1.0)

julia> @Btime convert(FixedDecimal{BigInt,1}, $(1))
  152.583 ns (9 allocations: 168 bytes)
FixedDecimal{BigInt,1}(1.0)

julia> @Btime convert(FixedDecimal{BigInt,2}, $(BigInt(1)))
  132.985 ns (8 allocations: 136 bytes)
FixedDecimal{BigInt,2}(1.00)

julia> @Btime convert(FixedDecimal{BigInt,2}, $(1))
  175.048 ns (10 allocations: 176 bytes)
FixedDecimal{BigInt,2}(1.00)
```

After:
```
julia> @Btime convert(FixedDecimal{BigInt,1}, $(BigInt(1)))
  2.000 ns (0 allocations: 0 bytes)
FixedDecimal{BigInt,1}(0.1)

julia> @Btime convert(FixedDecimal{BigInt,1}, $(1))
  28.684 ns (2 allocations: 40 bytes)
FixedDecimal{BigInt,1}(0.1)

julia> @Btime convert(FixedDecimal{BigInt,2}, $(BigInt(1)))
  132.382 ns (8 allocations: 136 bytes)
FixedDecimal{BigInt,2}(1.00)

julia> @Btime convert(FixedDecimal{BigInt,2}, $(1))
  131.776 ns (8 allocations: 136 bytes)
FixedDecimal{BigInt,2}(1.00)
```

Author: NHDaly

src/FixedPointDecimals.jl

@@ -308,6 +308,18 @@ function Base.convert(::Type{FD{T, f}}, x::Integer) where {T, f}
     end
     reinterpret(FD{T, f}, v)
 end
+function Base.convert(::Type{FD{BigT, f}}, x::Integer) where {BigT<:Union{BigFloat,BigInt}, f}
+    # We specialize on f==1, since julia can't eliminate BigInt multiplication.
+    if f == 1
+        # If x is already a BigInt, this is a no-op, otherwise we alloc a new BigInt.
+        return reinterpret(FD{BigT, f}, BigT(x))
+    end
+    # For the normal case, we multiply by C, which produces a BigInt value.
+    C = coefficient(FD{BigT, f})
+    # This can't throw since BigInt and BigFloat can hold any number.
+    v = x * C
+    reinterpret(FD{BigT, f}, v)
+end
 
 Base.convert(::Type{T}, x::AbstractFloat) where {T <: FD} = round(T, x)
 

test/FixedDecimal.jl

@@ -360,6 +360,12 @@ const UWFD2 = FixedDecimal{UInt128, 2}
     @test @allocated(Base.convert(FD2, Int128(-1))) == 0
 end
 
+@testset "BigInt conversion performance" begin
+    b = BigInt(2)
+    # Special-cased f=1 to not allocate for BigInt => FD conversion
+    @test @allocated(Base.convert(FixedDecimal{BigInt, 1}, b)) == 0
+end
+
 end  # module PerfTests
 
 @testset "promotion" begin