Work buffers for algorithms?

[gdalle]

See the discussion in #407

[rafaqz]

I like how Base.sort has sort! for in-place methods but also has a scratch keyword for the scratch space that some sorts like radix sort needs. Graphs could do something like that.

https://github.com/JuliaLang/julia/blob/75d5588d46bf7445626837f05e7a284ad85c7d30/base/sort.jl#L1730-L1739

And a helper method make_scratch:

  │    •  Base.Sort.make_scratch

  make_scratch(scratch::Union{Nothing, Vector}, T::Type, len::Integer)

  Returns (s, t) where t is an AbstractVector of type T with length at least len that is backed by the Vector s. If scratch !==
  nothing, then s === scratch.

  This function will allocate a new vector if scratch === nothing, resize! scratch if it is too short, and reinterpret scratch if its   
  eltype is not T.