[IMP] core: smaller and faster field cache

Consider a context-dependent field and two possible cache structures for
storing the data of that field for N records and K context keys.  The
common use-case is to assume that N is much larger than K.  We look at
how many dicts are used in each cache structure, where we label dicts
with N entries as "large", and dicts with K entries as "small".

     Structure                 | Number of dicts
    ---------------------------+-------------------
     cache[field][id][key]     | 1 large + N small
     cache[field][key][id]     | 1 small + K large

It is known that "small" dicts have a large relative overhead when
compared to "large" dicts.  Given this fact, the second cache structure
will generate less memory overhead.  Here are numbers for N=16, showing
the size in bytes taken by the cache structure.  Larger values of N lead
to larger size differences, always in favor of the second structure.

     Structure             |  K=1 |  K=2 |  K=3 |  K=4 |  K=5 |  K=6
    -----------------------+------+------+------+------+------+------
     cache[field][id][key] | 4488 | 4488 | 4488 | 4488 | 4488 | 6536
     cache[field][key][id] |  888 | 1536 | 2184 | 2832 | 3480 | 4256

Moreover, some memory operations are performed on many records and a
single context key.  Those operations are clearly simpler and faster
with the second cache structure.

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