Split-up core into Router and RouterPayable

[nkrishang]

Summary

Router and RouterPayable

We split-up the current core/Router into two separate options Router and RouterPayable. All presets in the /preset directory use Router.

Router does not have a payable fallback function, or a receive function. This allows for a receive function to be added as an extension, in contracts built on top of Router or one of the base router presets.

RouterPayable has a payable fallback function and a hardcoded receive function.

BaseRouter and BaseRouterWithDefaults

We split-up the current presets/BaseRouter into two separate options BaseRouter and BaseRouterWithDefaults.

BaseRouter no longer uses DefaultExtensionSet and is not meant to be initialized with default extensions in its constructor.

BaseRouterWithDefaults on the other hand inherits DefaultExtensionSet and can be initialized with default extensions in its constructor.