Near-complete rewrite of the PCIe-WB4 bridge.

Due to the way Altera uses padding, it is not possible to eliminate it without
the full state recovered during TLP decoding. Therefore, this version moves
the padding handling after the 64-32 conversion.

Furthermore, the decoding of TLP records now follows the PCIe 3.0 standard
instead of the slim Altera documentation. All incoming TLP formats should be
correctly processed without losing synchronization.