sat::Clause: Remove unused functions from the clause header.

Clause parsers were only used in unit tests and reader. Specialized functions
was not needed. So, these functions was not used.
Merge branch 'simplify_clause' into dev
Clean up CMakeLists.txt.

I removed build type switch from CMakeList.txt. Because we could switch build
type by cmake options.
If you want to compile for debug, do 'cmake -DCMAKE_BUILD_TYPE=DEBUG'.
If you want to compile for release, do 'cmake -DCMAKE_BUILD_TYPE=RELEASE'.
Change to pass CNF filename for unit tests from a variable in CMakeLists.txt.
Remove unused libraries from CMakeLists.txt.
test::SATFixture: Rewrite occurrence list construction not to use optional control.
Remove unused header from unit tests of CNF stream.
Rewrite unit tests of sat::OccurrenceMap not to use optional control.
Change sat::partial_erase to be noexcept.
sat::map: Change to throw nested exception.
Add forgotten header.
Add sat::make_basic_occurrence_list for acquisition.

sat::make_basic_occurrence_list(sat::Variable) did just simple allocation as
same as sat::BasicOccurrenceList's constructor. So I change to put simple
allocation to constructor and acquisition (allocation and mapping) to new
function.
sat::erase: Change to be passed only difference.

For splitting mapping and erasing, We are able to change it to noexcept.
Remove meaningless comments.
Remove meaningless exception check.
Change some functions to noexcept.
Remove meaningless comments.
Merge sat::mapOccurrence to sat::make_occurrence_map.

Old sat::make_occurrence_map just did simple allocation. I merge mapper into
maker and put simple allocation in constructor of sat::OccurrenceMap.
Fix error messages for sat::BasicOccurrenceList.
sat::VariableSequencer: Change to use default assign operators.