Add subdirectory for cryptography examples

+ Add example using columnar transposition for encrypting / decrypting messages
+ Fix cmake build error from duplicate lib-bst when building from project root

cpp/cryptography/columnar-transposition/CMakeLists.txt

@@ -0,0 +1,22 @@
+################################################################################
+## Author: Shaun Reed                                                         ##
+## Legal: All Content (c) 2021 Shaun Reed, all rights reserved                ##
+## About: A root project for practicing columnar transposition cipher in C++  ##
+##                                                                            ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0   ##
+################################################################################
+#
+
+cmake_minimum_required(VERSION 3.15)
+
+project(
+    #[[NAME]]   ColumnarTransposition
+    VERSION     1.0
+    DESCRIPTION "Practice implementing columnar transposition in C++"
+    LANGUAGES CXX
+)
+
+add_library(lib-cipher "lib-cipher.cpp")
+
+add_executable(columnar-transposition-test "driver.cpp")
+target_link_libraries(columnar-transposition-test lib-cipher)