Refactoring validation/decoding methods

[brunato]

In order to simplify the code and to make the validation faster i have the intention to refactoring the inner validation methods using decode/encode instead of generators (iter_decode/iter_encode).

The reason is that these generators returns a single value, eventually preceded by one or more validation errors. The code for iterating generators is more complex and slower.

Furthermore, the errors propagation is repeated at each level of the XML tree, slowing down the process.
The collecting of the errors (in 'lax' mode) will be handled with a list in kwargs (already used if provided).