Refactor transactions

Instead of creating one transaction for all operations and calling a
method on that transaction to add installs or removes, the operation
methods are constructors for transactions, and a transaction represents
the temprorary state on the filesystem of an operation on an individual
package or pair of packages.

Probably some convenience methods should be implemented for
Vec<Transaction> for easy comitting, but the idea is to expose the
stateful nature of the transactions to library users so that they can do
all the expensive parts of a large operation and commit all package
changes at once.