Fix and test strict keyword.

[evetion]

The strict query mode, while hard to use as its shadowed (#328), currently errors, as it tries to convert values back to a Missing Union.

This PR uses nonmissingtype for strict queries to make it work. It does mean that in cases where the query starts with a NULL value, it forces the declared type, potentially corrupting custom data (like the symbol in this test). Whether this should be the default behaviour is up for discussion and will not be changed in this PR. Also see the comment here:

SQLite.jl/src/SQLite.jl

Line 433 in ccdb8b0

# FIXME deserialized type have priority over declared type, is it fine?

I've also added a few type hints (found using Cthulhu) to fix type instabilities, for my large 200MB+ table, strict mode is now almost 10 times faster.