SRFI-159 is '(chibi show)' library from chibi scheme. Imported from
commit 5c43ca77 (move the col+ansi test to color section, 2019-11-07).

The library is restructured to become (srfi 159) (or (scheme
show)). This is mostly about sld changes:

chibi/show.sld          becomes srfi-159/internal/util.sld
chibi/show/base.sld     becomes srfi-159/internal/base.sld
chibi/show/color.sld    becomes srfi-159/color.sld
chibi/show/column.sld   becomes srfi-159/columnar.sld
chibi/show/pretty.sld   becomes srfi-159/internal/pretty.sld
chibi/show/unicode.sld  becomes srfi-159/unicode.sld

All .scm files keep their names from chibi except show.scm being
renamed to util.scm. monad.scm is new because we don't import
(chibi monad environment)
