made changes to (compile-lambda) and (compile-lambda-body) [fwiw] for 5.40
default compile-time environment for exercise 5.40
No, to avoid breaking backwards-compatibility, pass in the compile-time environment when invoking (compile) - say, from (compile-and-go)
Exercise 5.40 - extending the compile-time environment in (compile-lambda-body). It's only really like 2 new lines of code, but I had to spend quite a bit of time pondering what the RIGHT lines of code should be, as well as modifying ch5-compiler.scm in a backwards-compatible way. Unit tested, but not integration tested.