Fix immediate evaluation of lambda

Which we broke in the refactoring in 67b40823bf4a2efc5ef73d253f83d3e0db7a1503
Handle evaluation of #t and #f

So that they return themselves when evaluated rather than
nil. This fixes the 'describe (lat? l)' specs.
Merge pull request #6 from mocoso/tidy-up-after-meeting-5

Tidy up after meeting 5