[eval2] Move argument count checking into the VM.

This improves performance of function calls quite a lot.
[eval2] Fix the implementation of LET as opposed to LET*.
[eval2] Introduce a %FETCHARGS primitive for improved LAMBDA performance.
[eval2] Stack-allocate environments more aggressively.
[eval2] Fix the implementation of LET.
[eval2] Integrate %SET-ENVBOX into %WITH-ENVIRONMENT.
[eval2] Minor VM optimizations.
[eval2] Parameterize EVAL-LAMBDA over the function name for debugging/profiling purposes.
[eval2] Add a %TAGBODY primitive.

This greatly improves looping performance.
[eval2] Formatting.