feat(core): add cond-> and cond--> macros for conditional threading

[sqrew]

This PR adds the standard Clojure/Lisp cond-> and cond--> macros to Carp's standard library. These macros thread an expression through a series of forms, executing the forms only if their corresponding conditions evaluate to true.

Features:

• Handles odd number of clauses via a descriptive macro error.
• Works correctly with Carp's linear type system and borrow checker.
• Evaluates the initial expression exactly once and isolates intermediate bindings cleanly using gensyms.