v0.16.0 release with recent improvements

What's Changed

• improve guards information by @youkaichao in #30
• add freevars used in guard by @youkaichao in #31
• fix lazy_format_graph_code for nightly by @youkaichao in #33
• fix co_cellvars by @youkaichao in #35
• fix unpack sequence with 1 element by @youkaichao in #37
• update readme by @youkaichao in #38
• improve warning message by @youkaichao in #39

Full Changelog: v0.15.0...v0.16.0