Use mpq functions for Rational{BigInt}

[simonbyrne]

Fixes #9826.

A quick benchmark:

julia> X = [big(rand(1:10000)) // big(rand(1:10000)) for i = 1:100, j = 1:100];

julia> @time X*X; # before
 12.525924 seconds (45.29 M allocations: 1.267 GiB, 8.44% gc time)

julia> @time X*X; # after
  2.343117 seconds (18.95 M allocations: 733.016 MiB, 18.66% gc time, 17.26% compilation time)