Query transformer infrastructure & example query transformer implementations

[ARF1]

Based on visualfabriq#27 but can be rebased on master.

This introduces the infrastructure for plug-in query transformers. Included are three sample query transformers:

• InOperatorTransformer: my_col in ['ABC', 'DEF'] is transformed into (my_col == 'ABC') | (my_col == 'DEF'). The operation not in is similarly transformed.
• TrivialBooleanExpressionsOptimizer: (my_col == 'ABC') | (False) is transformed into False (limited usefulness without an intelligent query optimizer)
• CachedFactorOptimizer: converts comparisons containing columns with cached factors into comparisons using the factor instead. (Naive implementation, currently only useful for edge-cases.)

By default this PR does not change the behaviour or dependencies of bquery. Query transformers have to be explicitly enabled by configuring them, e.g.:

from transformers import InOperatorTransformer, TrivialBooleanExpressionsOptimizer
b.transformers = [InOperatorTransformer(), TrivialBooleanExpressionsOptimizer()]

For convenience, a shortcut is provided for these (currently) most useful transformers with transformers.standard_transformers:

from transformers import standard_transformers
b.transformers = standard_transformers

The overhead for queries is negligible for reasonably sized databases: For the query db["my_col=='AB1234567890'"] bquery without query transformers requires 362 ms, with all query transformers configured (including CachedFactorOptimizer) 367 ms.

With a non-compressed database the CachedFactorOptimizer shows some minor positive effects: 547 ms vs. 296 ms