[FileFormats.MPS] allow AbstractString when parsing MPS files

[odow]

x-ref #2888

Before

julia> @time main();
  6.228992 seconds (67.45 M allocations: 4.029 GiB, 16.71% gc time)

After

julia> @time main();
  5.684912 seconds (33.12 M allocations: 2.878 GiB, 16.25% gc time)

Code

function main()
    model = MOI.FileFormats.MPS.Model()
    MOI.read_from_file(model, "/Users/odow/Downloads/supportcase19.mps")
end

[odow]

Here's a small win. A better approach might be to completely refactor the parser into something that didn't read one line by line, split into string tokens, and then parse.