landed r7rs support

[Andy Wingo]

Hey all :)

Just a little heads-up that I just landed R7RS support.  Thanks to Göran
Weinholt for akku-scm (https://gitlab.com/akkuscm/akku-r7rs/) and
OKUMURA Yuki for yuni (https://github.com/okuoku/yuni), off of which
some of these files were based.  (These projects are public domain /
CC0).

The library syntax for R7RS is a subset of R6RS, so to use R7RS you just
(import (scheme base)) and off you go.  As with R6RS also, there are
some small lexical incompatibilities regarding hex escapes; see "R7RS
Incompatibilities" in the manual.  Also there is a --r7rs command-line
option.

Cheers,

Andy