Export and document emptymutable

[andyferris]

The empty API occassionally gets used defined immutable containers, like StaticArrays' SVector and MVector. Internally, we use Base.emptymutable to construct arrays (and sets/dicts) that we are about to use push! etc to modify. I think it would be healthy if we:

1. Document what emptymutable is, how it differs to empty, and how it gets used internally
2. Export and maintain it with the expectation that external users will call and extend it

From an external point of view, emptymutable can seem like an unstable, internal API that shouldn't be leveraged, but I feel the status quo gives the wrong impression. See e.g. JuliaData/SplitApplyCombine.jl#54 (and related JuliaArrays/StructArrays.jl#238).