Ambiguous conversions don't cause overload resolution to pick a different overload.
Add another explicit conversion test.
More implicit conversion unit tests.
Merge branch 'upstream_master' into new-user-defined-operator-try

Conflicts:
	ICSharpCode.NRefactory.CSharp/Resolver/CSharpConversions.cs
	ICSharpCode.NRefactory.Tests/CSharp/Resolver/ExplicitConversionsTest.cs
Look at implicit and explicit user-defined conversion operators at the same time when performing an explicit conversion