Add some of the easier/smaller R6RS libraries

lib/rnrs/control.sld

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+(library (rnrs control)
+  (export when unless
+          do
+          case-lambda)
+  (import (scheme base)
+          (scheme case-lambda)))

lib/rnrs/eval.sld

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+(library (rnrs eval)
+  (export eval
+          environment)
+  (import (scheme eval)))

lib/rnrs/mutable-pairs.sld

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+(library (rnrs mutable-pairs)
+  (export set-car! set-cdr!)
+  (import (scheme base)))

lib/rnrs/mutable-strings.sld

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+(library (rnrs mutable-strings)
+  (export string-set!
+          string-fill!)
+  (import (scheme base)))

lib/rnrs/programs.sld

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+(library (rnrs programs)
+  (export command-line
+          exit)
+  (import (scheme process-context)))

lib/rnrs/sorting.sld

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+(library (rnrs sorting)
+  (export (rename list-stable-sort list-sort)
+          (rename vector-stable-sort vector-sort)
+          (rename vector-stable-sort! vector-sort!))
+  (import (srfi 132)))