Add support for -not and -as

Forgot to put in support for `-not`, and `-as` was throwing a NRE
because of a method resolution error on my part.

Author: SeeminglyScience

src/PSLambda/CompileVisitor.cs

@@ -916,6 +916,8 @@ public object VisitUnaryExpression(UnaryExpressionAst unaryExpressionAst)
                     return Assign(child, Increment(child));
                 case TokenKind.PostfixMinusMinus:
                     return Assign(child, Decrement(child));
+                case TokenKind.Not:
+                    return Not(PSIsTrue(child));
                 default:
                     ReportNotSupported(
                         unaryExpressionAst.Extent,

src/PSLambda/ReflectionCache.cs

@@ -61,7 +61,7 @@ internal static class ReflectionCache
                     var parameters = method.GetParameters();
                     return parameters.Length == 2
                         && parameters[0].ParameterType == typeof(object)
-                        && parameters[1].ParameterType.IsGenericParameter;
+                        && parameters[1].ParameterType.IsByRef;
                 },
                 null)
                 .FirstOrDefault();