Add the SQLITE_NOTNULL P5 code for comparison operations - really a composite
of SQLITE_NULLEQ and SQLITE_JUMPIFNULL.  This flag indicates that NULL operands
are not possible and raises and assert() if NULL operands are seen.
Also omit an unnecessary scan of the sqlite_sequence table when writing
into an AUTOINCREMENT table.
Add the VdbeCoverageNeverTaken() macro, and comments that better describe how
the VDBE branch coverage measurement works.  Add some tags to provide 100%
VDBE branch coverage.
Make sure that the difference between two string pointers is passed into
sqlite3_mprintf() as type "int".
Fix a harmless shadowed-variable warning.
Fix the text of a comment used to generate VDBE opcode documentation so that
it omits symbols that can be mistaken for a hyperlink by the parser.