Faster ReadString by avoiding memory allocations in []byte to string conversion

[saurabhagrawal-86]

Benchstat results:

name old time/op new time/op delta _jsoniter_large_file 10.8µs ± 6% 8.9µs ± 7% -17.69% (p=0.000 n=10+10) _jsoniter_large_file-2 10.6µs ±10% 9.2µs ± 5% -13.07% (p=0.000 n=10+10) _jsoniter_large_file-4 10.9µs ±10% 9.2µs ± 9% -14.91% (p=0.000 n=10+10) name old alloc/op new alloc/op delta _jsoniter_large_file 4.91kB ± 0% 4.37kB ± 0% -11.07% (p=0.000 n=10+10) _jsoniter_large_file-2 4.91kB ± 0% 4.37kB ± 0% -11.07% (p=0.000 n=10+10) _jsoniter_large_file-4 4.91kB ± 0% 4.37kB ± 0% -11.07% (p=0.000 n=10+10) name old allocs/op new allocs/op delta _jsoniter_large_file 78.0 ± 0% 5.0 ± 0% -93.59% (p=0.000 n=10+10) _jsoniter_large_file-2 78.0 ± 0% 5.0 ± 0% -93.59% (p=0.000 n=10+10) _jsoniter_large_file-4 78.0 ± 0% 5.0 ± 0% -93.59% (p=0.000 n=10+10) 

[saurabhagrawal-86]

Unsafe is not really safe here.