srfi-159: add missing pretty printing procedures

like the rest of srfi-159, this is coming straight from the chibi reference
implementation

furthermore, restrict export of pretty-shared and written-shared since they are
part of srfi-166

src/std/build-deps

@@ -397,12 +397,22 @@
  (std/srfi/159/color
   (gxc: "srfi/159/color" (extra-inputs: ("srfi/159/color.scm")))
   (gerbil/core std/srfi/13 std/srfi/159/base))
+ (std/srfi/159/pretty
+  (gxc: "srfi/159/pretty" (extra-inputs: ("srfi/159/pretty.scm")))
+  (gerbil/core
+   std/srfi/1
+   std/srfi/125
+   std/srfi/130
+   std/srfi/159/base
+   std/srfi/159/show
+   std/srfi/159/string))
  (std/srfi/159
   "srfi/159"
   (gerbil/core
    std/srfi/159/base
    std/srfi/159/color
    std/srfi/159/columnar
+   std/srfi/159/pretty
    std/srfi/159/show
    std/srfi/159/unicode))
  (std/srfi/160/macros

src/std/build-spec.ss

@@ -98,6 +98,7 @@
     "srfi/159/columnar"
     (gxc: "srfi/159/unicode" (extra-inputs: ("srfi/159/unicode.scm")))
     (gxc: "srfi/159/color" (extra-inputs: ("srfi/159/color.scm")))
+    (gxc: "srfi/159/pretty" (extra-inputs: ("srfi/159/pretty.scm")))
     "srfi/159"
     "srfi/160/cvector"
     "srfi/160/base"

src/std/srfi/159.ss

@@ -6,9 +6,14 @@
         ./159/show
         ./159/columnar
         ./159/unicode
-        ./159/color)
-(export (import: ./159/base
-                 ./159/show
-                 ./159/columnar
-                 ./159/unicode
-                 ./159/color))
+        ./159/color
+        ./159/pretty)
+(export
+  (except-out (import: ./159/base
+                       ./159/show
+                       ./159/columnar
+                       ./159/unicode
+                       ./159/color
+                       ./159/pretty)
+              written-shared
+              pretty-shared))