Refactor tail call resolution logic

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.

examples/err.myc

@@ -0,0 +1,6 @@
+;; Output: 0
+(do
+  (def history (series :float))
+  (push history 0.0)
+  (history 0)
+)

examples/sma.myc

@@ -10,6 +10,7 @@
               (def sum (+ val (history 0)))
 			  (push history sum)
               (/ sum length)
+(history 0)
            ))
       )))