Automated parsabilety of hex numbers ("0x...") (#35)

* Automated parsabilety of hex numbers starting with "0x"

When the NM client parses something like a Dictionary from JSON where input keys look like this:
"0x0000000000000000000000000000000000000000000000000000000000000000"
we currently fail. To fix this, we need to re-implement the dictionary parser as they do here (https://stackoverflow.com/a/7010231/1973207 )  with explicit `(UInt256)BigInteger.Parse(skey.Substring(2), NumberStyles.HexNumber);` in it. This would help automate this and allow the elimination of specialised dictionary parsers like that.

Author: OlegJakushkin

src/Nethermind.Int256.Test/UInt256Tests.cs

@@ -1,4 +1,5 @@
-using System;
+using System;
+using System.Globalization;
 using System.Numerics;
 using FluentAssertions;
 using NUnit.Framework;
@@ -469,5 +470,13 @@ string Expected(string valueString)
                 catch (Exception e) when (e.GetType() == expectedException) { }
             }
         }
+        [Test]
+        public virtual void ParseLongNumber()
+        {
+            string hexValueWith66Zeroes = "0x" + new string('0', 66);
+            BigInteger bigIntWith66Zeroes = BigInteger.Parse(hexValueWith66Zeroes.Substring(2), NumberStyles.HexNumber);
+            var UintParsedValue = UInt256.Parse(hexValueWith66Zeroes);
+            Assert.AreEqual((UInt256)bigIntWith66Zeroes, UintParsedValue);
+        }
     }
 }

src/Nethermind.Int256/UInt256.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Buffers.Binary;
 using System.Globalization;
 using System.Numerics;
@@ -1882,9 +1882,11 @@ public void Convert(out BigInteger big)
 
         public static UInt256 Parse(in ReadOnlySpan<char> value, NumberStyles numberStyles) => !TryParse(value, numberStyles, CultureInfo.InvariantCulture, out UInt256 c) ? throw new FormatException() : c;
 
-        public static bool TryParse(string value, out UInt256 result) => TryParse(value, NumberStyles.Integer, CultureInfo.InvariantCulture, out result);
+        public static bool TryParse(string value, out UInt256 result) => TryParse(value.AsSpan(), out result);
 
-        public static bool TryParse(ReadOnlySpan<char> value, out UInt256 result) => TryParse(value, NumberStyles.Integer, CultureInfo.InvariantCulture, out result);
+        public static bool TryParse(ReadOnlySpan<char> value, out UInt256 result) => value.StartsWith("0x", StringComparison.OrdinalIgnoreCase)
+            ? TryParse(value.Slice(2), NumberStyles.HexNumber, CultureInfo.InvariantCulture, out result)
+            : TryParse(value, NumberStyles.Integer, CultureInfo.InvariantCulture, out result);
 
         public static bool TryParse(string value, NumberStyles style, IFormatProvider provider, out UInt256 result) => TryParse(value.AsSpan(), style, provider, out result);