Avoid creating new array

[spirinvladimir]

Tiny performance improvement with checking first two chars with '0' instead of checking new string wiht '00'

According MDN:
Array.prototype.slice()
The slice() method returns a shallow copy of a portion of an array into a new array object selected from start to end (end not included) where start and end represent the index of items in that array.

Performance tests:

> f_01 = n => {var x=0;while(n--) {var s=String(Math.random()*10); if (s[0] === '0' && s[1] === '0') x++}return x};
[Function: f_01]
> f_slice = n => {var x=0;while(n--) {var s=String(Math.random()*10); if (s.slice(0,2) === '00') x++}return x};
[Function: f_slice]
> n=1e6;console.time(n);f_slice(n);console.timeEnd(n);
1000000: 283.498ms
undefined
> n=1e6;console.time(n);f_01(n);console.timeEnd(n);
1000000: 261.283ms
undefined
> n=1e8;console.time(n);f_slice(n);console.timeEnd(n);
100000000: 25.068s
undefined
> n=1e8;console.time(n);f_01(n);console.timeEnd(n);
100000000: 23.241s
undefined

[coveralls]

Coverage remained the same at 90.598% when pulling 5ebbfea on spirinvladimir:patch-1 into 75f3981 on ethereumjs:master.

[ryanio]

hey @spirinvladimir, thanks for the PR! seems reasonable to me