Alternative to creating identity containers: Just never merge heterogenous inputs

[kristovatlas]

Relevant attack:

Link identities by observing that addresses associated with both identities are used as inputs in the same transaction

1 Countermeasure:

void constructing transactions that contain inputs from more than one identity/account

This would be a new countermeasure

[kristovatlas]

This came up in discussion with @dcousen about how coinswaps could avoid merging heterogenous inputs in a single tx in relation to #44