Avoid consing for optional procedure arguments

[sjamaan]

Currently, anything that accepts optional, rest or keyword arguments has to build a list and then pick that list apart. This is wasteful because it should not be necessary at all.

We could start with the low-hanging fruit like case-lambda and #!optional, where the binding of arguments and argument count checking is relatively simple. Handling #!key and #!rest or dotted tail rest argument notation is something that seems to me either too difficult and possibly not even worth it: you usually want to treat those as a list (but patterns like optional could still be handled specially, if nothing else refers to the identifier).

[evhan]

Moving to 5.0.0 just to keep the listings tidy.

[sjamaan]

Patch against e25965

[sjamaan]

Final version including explanatory commit message sent to the mailing list

[felix winkelmann]

Final version has been applied.

[sjamaan]

This has been implemented for #!optional, let-optionals* and also case-lamdba. There might be room for improvement (as always), but for now I feel we've done enough; the approach we use even makes manual accesses of rest arguments optimisable, depending on the exact pattern used, of course.