Query optimisation

[Chris Muller]

Today I noticed a performance bug; single-clause queries do not need
to go through a (expensive) routine needed only for multi-clause
queries.

A simple guard in the code has now sped up single-clause queries by 100 times.

A single-clause query is one which has no more than one "range" of
keys specified, like:

  events where:
    [ : each |
    each date
      between: '1/1/2006' asDate
      and: '12/31/2006' asDate ]

Like in older Magma (pre-queries) this is really fast again.