bring back readable boolean literals

[alexshinn]

Scheme used to use #!true and #!false before
abbreviating to the unfortunate #t and #f syntax,
which look far too alike.

We probably can't remove these, but we could
add them back in as aliases (optionally without
the "!") and hope to remove the short versions
in R8RS.

[cowan]

WG1 accepted this proposal.