New update semantics for `..` (recursion).

[01mf02]

This PR changes the semantics of .. on the left-hand side of updates.

# `.. |= f` expands to ...
def recurse: ., (.[]? | recurse); recurse |= f   # before
def recurse: (.[]? | recurse), .; recurse |= f   # after

This prevents stack overflows that could previously occur (#139, #235).