Implemented two-fer exercise

exercises/two-fer/README.md

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+`Two-fer` or `2-fer` is short for two for one. One for you and one for me.
+
+```text
+"One for X, one for me."
+```
+
+When X is a name or "you".
+
+If the given name is "Alice", the result should be "One for Alice, one for me."
+If no name is given, the result should be "One for you, one for me."

exercises/two-fer/example.scm

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+(define-module (two-fer)
+  #:export (two-fer))
+
+(define two-fer
+  (lambda* (#:optional name)
+    (let ((target (or name "you")))
+      (string-concatenate (list "One for " target ", one for me.")))))

exercises/two-fer/two-fer-test.scm

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+;; Load SRFI-64 lightweight testing specification
+(use-modules (srfi srfi-64))
+
+;; Suppress log file output. To write logs, comment out the following line:
+(module-define! (resolve-module '(srfi srfi-64)) 'test-log-to-file #f)
+
+;; Require module
+(add-to-load-path (dirname (current-filename)))
+(use-modules (two-fer))
+
+(test-begin "two-fer")
+
+(test-assert "no name given"
+             (equal? (two-fer)
+                     "One for you, one for me."))
+
+(test-assert "a name given"
+             (equal? (two-fer "Alice")
+                     "One for Alice, one for me."))
+
+(test-assert "another name given"
+             (equal? (two-fer "Bob")
+                     "One for Bob, one for me."))
+
+(test-end "two-fer")

exercises/two-fer/two-fer.scm

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+(define-module (two-fer)
+  #:export (two-fer))