[FileFormats.LP] fix type stability of LP reader

[odow]

Another follow-up to #2840

Now reading into an MOI model is only 4x slower than just counting the new lines char-by-char:

julia> import MathOptInterface as MOI

julia> function main()
           model = MOI.FileFormats.LP.Model{Float64}()
           MOI.read_from_file(model, "/Users/odow/Downloads/QPLIB_10005.lp")
           return model
       end
main (generic function with 1 method)

julia> @time main();
  1.065072 seconds (43.71 M allocations: 1.511 GiB, 22.66% gc time)

julia> function count_newlines(io::IO)
           i = 0
           while !eof(io)
               if read(io, Char) == '\n'
                   i += 1
               end
           end
           return i
       end
count_newlines (generic function with 1 method)

julia> @time open(count_newlines, "/Users/odow/Downloads/QPLIB_10005.lp", "r")
  0.274165 seconds (14 allocations: 792 bytes)
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