round (and trunc, floor, ceil) not defined from `Float64` or `Float32` to `BigInt`

[carlobaldassi]

I didn't see this reported elsewhere, so:

julia-0.4> round(BigInt, 2.5)
ERROR: MethodError: `trunc` has no method matching trunc(::Type{BigInt}, ::Float64)
Closest candidates are:
  trunc{T<:Integer}(::Type{T<:Integer}, ::Integer)
  trunc(::Type{Signed}, ::Float64)
  trunc(::Type{Unsigned}, ::Float64)
  ...
 in round at float.jl:177

There are definitions going from BigFloat to BigInt. Maybe we could use those, if implementing a direct route is unfeasible.

julia-0.4> round(BigInt, big"2.5")
2

[simonbyrne]

Ha. I spent a lot of effort getting things correct the other way, but it didn't occur to me that we missed these.

It should be possible to do this directly using mpz_set_d without going via BigFloats.