[MOD] Enabled tail-call optimization for dynamic functions.
[MOD] Optimized `fn:subsequence(..)` and sequence casts.
[MOD] `Expr#markTailCalls()` does not replace expressions any more.
[ADD] Test case for TCO on function items.
Merge pull request #749 from LeoWoerteler/general-tco

Tail-Call Optimization for Dynamic Functions