Bumped copyright to 2021

Fix for GitHub Actions: include unannotated tags in 'git describe'

GitHub Actions can release binaries

Migrate CI to GitHub Actions

Added len() and reverse() for String

First pass at state registers

Added mean(), variance(), and stddev()

Reset sema's scope if an error occurs

First pass at lineage

Added stream_load() as a stub to test streaming operations

Fixed bug when overloading placeholder generics

Fixed bug in how generic placeholders work with inferred Dataframe types

Added generic placeholders

Added documentation via '?'

Added reverse()