Support for transient variables

[karnkaul]

It would be useful to be able to contextualize - ie, make available as defined constants - certain variables for specific evaluations.

A few viable approaches:

• A new RAII type that represents a constant defined in an Env, "undefining" it on destruction.
  • Gotchas: lifetime and dangling references. (Since the type would need to store a reference to Env.)
• Env storing a pointer to a parent, delegating symbol lookup to it if not found in its own table. The transient variables would be defined in a child Env which would be used for desired evaluations.
  • Gotchas: while root Env should define builtins in its constructor, child ones shouldn't. Solution: separate root and child Envs into distinct (related) types; leverage inheritance for code reuse.