default offset argument for mmap / anonymous does not work

[chethega]

Small convenience issue:

julia> using Mmap

julia> anon = Mmap.Anonymous()
Mmap.Anonymous("", false, true)

julia> Mmap.mmap(anon, Vector{UInt8}, (4,))
ERROR: MethodError: no method matching position(::Mmap.Anonymous)
Closest candidates are:
  position(::Base.SecretBuffer) at secretbuffer.jl:154
  position(::Base.Filesystem.File) at filesystem.jl:225
  position(::Base.Libc.FILE) at libc.jl:101
  ...
Stacktrace:
 [1] mmap(::Mmap.Anonymous, ::Type{Array{UInt8,1}}, ::Tuple{Int64}) at /build/julia/src/julia-1.1.0/usr/share/julia/stdlib/v1.1/Mmap/src/Mmap.jl:186
 [2] top-level scope at none:0

julia> Mmap.mmap(anon, Vector{UInt8}, (4,), 0)
4-element Array{UInt8,1}:
 0x00
 0x00
 0x00
 0x00

I'd guess we should either implement position(::Mmap.Anonymous)=0 or change the default argument, like e.g.

function mmap(io::IO,
              ::Type{Array{T,N}}=Vector{UInt8},
              dims::NTuple{N,Integer}=(div(filesize(io)-position(io),sizeof(T)),),
              offset::Integer=(io isa Anonymous ? 0 : position(io)); grow::Bool=true, shared::Bool=true) where {T,N}