Finalize NULL related decisions

[mtoy-googly-moogly]

1. = NULL and != NULL are deprecated
2. IS NULL and IS NOT NULL are official
3. != and !~ return true if either argument is null
4. not x returns true if x is null
5. Added partials for is null and is not null for the sake of pick statements
6. Tests for all the above

• Fix docs for = NULL and IS NULL ( Update docs and samples to prefer IS NULL malloydata.github.io#211 )
• Write a page about why we do this in the docs tree
• is null and is not null as legal partials ?