Optimise expand using min weight techniques

[olly]

ESet::Internal::expand() and ExpandWeight? could optimise using min weight techniques like those used by the matcher - for example, ExpandWeight? could provide an upper bound and expand() could use it to know when further terms can't make it in into the ESet.

The gain is probably much less than for the matcher, since termlists aren't chunked, so we take the I/O hit regardless, but this could save a fair bit of CPU so is still useful.