wkb: Implement __load_multilinestring

There is a TODO here that needs to be taken care of to do parsing
validation checks.
wbk_test: Add multilinestring `loads` test cases
wkb:Add validity asserts to __load_multilinestring

Each linestring contained in a multilinestring is now checked that it
has the proper WKB type and endianness header.

This also fixes some PEP8 errors about unused variables.