refactor implementation of environments

[dherman]

• Need a current-lexical-scope? parameter
• Need a contains-direct-eval? predicate for each function
• When current-lexical-scope? is false, must use explicit scope chain
• Features that affect current-lexical-scope?
  • with
  • direct eval
• Implementation of eval will have to fake its environment
  • PLT won't allow inheriting
  • so implement with current-lexical-scope? set to false
  • this fits with my (admittedly rude and obnoxious) "punish with and eval" implementation philosophy

See also #50, #51, #54.

[dherman]

Also need a notion of a lexical frame, i.e., any form which introduces a new rib in the environment

• function
• let statement/expression
• block containing let declarations

[dherman]

Done in 7:5.