Generic buffer API

[axic]

Primea has introduced a generic buffer API where each buffer is referred to by an index and helpers exists to query the size and retrieve the buffer.

We could consider to use this instead of the collection of calldata*, retundata*, code*, externalcode*. We'd still need to have a way to retrieve a message index, for example a helper for calldatamessageindex or it could be passed on to the function. For return data an index could be returned by call*.