The tail call resolution logic was duplicated in `Environment::call` and
`VM::run`. This commit extracts the tail call resolution logic into a
single method `VM::resolve_tail_calls` and uses it in both places.
Additionally, this commit adds support for series indexing as a form of
tail call, allowing for direct access to series elements through the
`series(index)` syntax. This is useful for back-referencing in
time-series data.
A new example `err.myc` is added to demonstrate basic series usage and
error handling.
Michael Schimmel
13dc6beb52