Provide syntax sugar for common queries (Accept, GetPropertiesAndFields)

This PR implements _some_ of the features discussed in #43.

These tasks are planned for this PR, introducing `ReflectionElementEnvy`:
- [x] Accept{T}(this IEnumerable{IReflectionElement}, IReflectionVisitor{T})
- [x] GetPropertiesAndFields(BindingFlags) - simplifies querying for properties and fields at the same time.
- [x] Rename `ReflectionElementExtensions` --> `ReflectionElementEnvy`