Simplify comprehension compilation in v3_core

Given that map comprehensions and generators and maybe && generators will be added
to the language in the future, v3_core:lc_tq and v3_core:bc_tq to avoid a
complexity explosion, where there are as many related clauses as the product of
the number of types of generators and the number of types of comprehensions.

The new code abstract over all generators at the same time, there is only one clause
for generators per type of comprehension, and all the filter code has been put
in a common function filter_tq.