Support unsafe_trunc(T, ::Integer)

[timholy]

The input types supported by unsafe_trunc and rem are currently disjoint:

julia> unsafe_trunc(Int32, 3.2)
3

julia> 3.2 % Int32
ERROR: MethodError: no method matching rem(::Float64, ::Type{Int32})
Closest candidates are:
  rem(::Bool, ::Type{Int32}) at int.jl:221
  rem(::Int16, ::Type{Int32}) at int.jl:218
  rem(::Int64, ::Type{Int32}) at int.jl:208
  ...

julia> unsafe_trunc(Int32, 3)
ERROR: MethodError: no method matching unsafe_trunc(::Type{Int32}, ::Int64)
Closest candidates are:
  unsafe_trunc(::Type{Int32}, ::Float32) at float.jl:159
  unsafe_trunc(::Type{Int32}, ::Float64) at float.jl:160
  unsafe_trunc{T<:Integer}(::Type{T<:Integer}, ::BigFloat) at mpfr.jl:130

julia> 3 % Int32
3

I'm not sure this is a bug, but it seems a bit inconvenient. Since trunc(Int32, 3) works, I thought it would be best to define unsafe_trunc for integer inputs.

[tkelman]

check type equality too with === ?