[mono-api-html] Take arity into account when comparing methods.

[mono-api-html] Take arity into account when comparing methods.

Fixes a crash when there are two methods overloaded by arity:

System.InvalidOperationException: Sequence contains more than one matching element
  at System.Linq.Enumerable.Single[XElement] (IEnumerable`1 source, System.Func`2 predicate, Fallback fallback) [0x00038] in /private/tmp/source-mono-mac-3.10.0-branch/bockbuild-mono-3.10.0-branch/profiles/mono-mac-xamarin/build-root/mono-3.10.0/mcs/class/System.Core/System.Linq/Enumerable.cs:2371
  at System.Linq.Enumerable.SingleOrDefault[XElement] (IEnumerable`1 source, System.Func`2 predicate) [0x00007] in /private/tmp/source-mono-mac-3.10.0-branch/bockbuild-mono-3.10.0-branch/profiles/mono-mac-xamarin/build-root/mono-3.10.0/mcs/class/System.Core/System.Linq/Enumerable.cs:2445
  at Xamarin.ApiDiff.MemberComparer.Find (IEnumerable`1 target) [0x00019] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/MemberComparer.cs:72
  at Xamarin.ApiDiff.MemberComparer.Compare (IEnumerable`1 source, IEnumerable`1 target) [0x0003c] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/MemberComparer.cs:83
  at Xamarin.ApiDiff.MemberComparer.Compare (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target) [0x000a2] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/MemberComparer.cs:55
  at Xamarin.ApiDiff.ClassComparer.Modified (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target) [0x0005b] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/ClassComparer.cs:210
  at Xamarin.ApiDiff.Comparer.Compare (IEnumerable`1 source, IEnumerable`1 target) [0x00093] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/Comparer.cs:80
  at Xamarin.ApiDiff.ClassComparer.Compare (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target) [0x00055] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/ClassComparer.cs:67
  at Xamarin.ApiDiff.NamespaceComparer.Modified (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target) [0x0001a] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/NamespaceComparer.cs:75
  at Xamarin.ApiDiff.Comparer.Compare (IEnumerable`1 source, IEnumerable`1 target) [0x00093] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/Comparer.cs:80
  at Xamarin.ApiDiff.NamespaceComparer.Compare (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target) [0x00055] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/NamespaceComparer.cs:49
  at Xamarin.ApiDiff.AssemblyComparer.Modified (System.Xml.Linq.XElement source, System.Xml.Linq.XElement target) [0x0002b] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/AssemblyComparer.cs:70
  at Xamarin.ApiDiff.Comparer.Compare (IEnumerable`1 source, IEnumerable`1 target) [0x00093] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/Comparer.cs:80
  at Xamarin.ApiDiff.AssemblyComparer.Compare () [0x0004a] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/AssemblyComparer.cs:50
  at Xamarin.ApiDiff.Program.Main (System.String[] args) [0x00328] in /Users/builder/data/lanes/macios-mac-master/177810cb/source/mono/mcs/tools/corcompare/mono-api-html/ApiDiff.cs:164