Sort ExportedType table when writing to comply with new ECMA Augments

[ElektroKill]

fixes #558

The algorithm suggested in the issue was adjusted to allow more than one nested type per exported type.

Something to consider:
The current algorithm does not dispose of the enumerators generated just like the method in

dnlib/src/DotNet/AllTypesHelper.cs

Line 14 in 4e0837c

public static IEnumerable<TypeDef> Types(IEnumerable<TypeDef> types) {

. This should be fine since list enumerators don't contain any logic in their Dipose method but this is something we'd probably need to add to both implementations for future-proofing and potential reuse. Perhaps we could also extract this traversal into an additional class for future use.

[wtfsck]

Thanks, added an issue about disposing of enumerators: #560