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 could use a rewrite
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.

It should also be noted that generator inputs are now compiled before the rest
of the qualifiers, reversing names of nested comprehensions.