FIX: IO#read create a large number of objects.

same as #32 but don't use `String#<<`.

before:
```
% time ./bin/mruby -e 'File.open("/dev/zero").read(10000000); system "grep VmData /proc/#{$$}/status"' 
VmData:	  264168 kB
./bin/mruby -e   1.79s user 2.21s system 99% cpu 3.999 total
```
after:
```
% time ./bin/mruby -e 'File.open("/dev/zero").read(10000000); system "grep VmData /proc/#{$$}/status"'
VmData:	   27264 kB
./bin/mruby -e   0.00s user 0.02s system 98% cpu 0.020 total
```