Add a default implementation of regenerate in terms of generate and project

[marcoct]

Add the requirement that

q(t; x, u) = q(t; x, u|_{dom t})

Practically speaking, this means that sampling from q should never need to read the values of addresses from u that it doesn't end up putting in the trace. This is indeed the case in ancestral sampling.

The motivation for making this restriction is that it makes it possible to provide a generic default implementation of regenerate in terms of generate and project. This implementation does not exploit incremental computation, but it is nice that there is a default one.

1. given initial choice map t, initial args, x, new args, x', selection set A
2. construct choice map u that is the restriction of t to the complement of A
3. run generate(f, x', u), obtaining new choice map t' and weight w'
4. B = dom(t') setminus I
5. w = project(t, B)
6. return t', weight=w'/w