Use :buffer_in and turn off errno saving to speed up passing string data via FFI

This (at least on my machine) makes JRuby 1.7.3 FFI almost as fast as ruby-2.0.0 with the cext (slower on 32, faster on 64):

    benchmarking for 10000000 iterations
                         user     system      total        real
    JRuby:
        ffi FNV 32:          6.610000   0.140000   6.750000 (  6.879000)
        ffi FNV 64:          6.280000   0.040000   6.320000 (  6.540000)

    MRI:
        ffi FNV 32:          8.670000   0.030000   8.700000 (  8.844798)
        ffi FNV 64:         12.660000   0.050000  12.710000 ( 12.863284)
        c-ext FNV 32:        5.560000   0.020000   5.580000 (  5.641636)
        c-ext FNV 64:        9.470000   0.030000   9.500000 (  9.697421)

btw, when benchmarking JRuby, always use '--server' and bump up the iterations - ideally, do a million reps for warmup before running the actual benchmark.