[R6RS] Safe/unsafe mode

[dyb at cs.indiana.edu]

I have checked in a new version of safety.txt.  Because I wasn't not sure
whether Will would agree with the changes, I called my version safety2.txt
and left safety.txt alone.

I feel that safety.txt puts Proposal 3 in a bad light.  In safety2.txt, I
have reworded the descriptions of Proposal 3 and the examples section to
describe the proposal in a way that I believe is more even-handed. 
Fortunately, safety.txt does not put the other proposals in a bad light,
so I was able to leave their descriptions alone for the most part, but I
did mention for them what (I understand) the macro expander must do with
declarations.  I have also added a couple of additional examples.

At this point, FYI, my first choice is Proposal 1, my second is Proposal
3, third is Proposal 0, and fourth is Proposal 2.

Kent