Make test for lambda pass

We assume that if a list starts with a list, then that list is a
lambda. And we immediately execute it.
Some better variable names
Replace if/else with case statement
Allow storing lambdas in the environment

Add a simple test to help drive this before tackling the more
complicated `lat?` test.

Assumptions:
 - user-defined functions can override language-defined functions
 - dynamic scope, that is we are merging environments that are passed in
 to lambda.
Remove duplication in immediate evaluation of lambda
s/parameter_name/parameter
Merge pull request #5 from chrislo/310314-meeting

Fifth meeting