make stringification a bit more efficient
this avoids an extra copy of the data. example:
before:
julia> @time for i=1:10^6; string(1.5); end
0.829682 seconds (8.00 M allocations: 335.702 MB, 8.11% gc time)
after:
julia> @time for i=1:10^6; string(1.5); end
0.448034 seconds (7.00 M allocations: 259.399 MB, 4.37% gc time)
