Design change: use a single Adapter per each ERC20/xERC20 pairs

[gitmp01]

This design change avoids the need of the XERC20Registry contract.
The registry approach has raised the following problems:

• Ownership: can't be transferred to token owners, since it may raise a censorship problem. Also it can't be renounced entirely because noone can register pairs anymore.
• Registration: apply a restriction of a given pair only to the ERC20 owner has been taken into account, however it isn't possible to get the owner of a ERC20 on chains where it isn't deployed (the registry is meant to be deployed on each supported chain).

This has been avoided by setting the pairs in the Adapter's constructor, the main disadvantage is that the dApp has to keep track of each official Adapter address for each pair in order to interact with them.