Handle bitwise operations on Enums

[metoule]

The Linq expression trees don't resolve the binary operators on Enums. We work around that by converting each operand to the underlying type (e.g. integers), performing the binary operation, then converting it back to the enum type:

// from 
var res = RegexOptions.Compiled | RegexOptions.Singleline;
// to 
var res = (RegexOptions)((int)RegexOptions.Compiled | (int)RegexOptions.Singleline);

Example:

var target = new Interpreter();
target.Reference(typeof(RegexOptions));
target.Reference(typeof(DateTimeKind));

var result = target.Eval<RegexOptions>("RegexOptions.Compiled | RegexOptions.Singleline");
Assert.True(result.HasFlag(RegexOptions.Compiled));
Assert.True(result.HasFlag(RegexOptions.Singleline));

Fixes #235

[davideicardi]

LGTM! Thanks!