Optimize state key conversion

[greymistcube]

When converting an Address to state key, we convert the given Address to a lower hex string.

I've benchmarked the following methods:

Method 1:

ToStateKey(Address address) => address.ToHex().ToLowerInvariant();

Method 2:

ToStateKey(Address address) => ByteUtil.Hex(address);

Method	Mean	Error	StdDev
Method1	257.97 ms	5.712 ms	16.480 ms
Method2	38.80 ms	0.775 ms	1.857 ms

As we can see, it is at least 6 times faster. 🙃

As ByteUtil.Hex() already outputs a lower cased hex string, there is no need to "lower" post-checksum. Also, in the case above ToLower() would suffice as hex representation is culture insensitive. 🙄

Also, as a side note, we should be more specific with the method documentation for ByteUtil.Hex() that it specifically outputs a lower cased hex string. 😗