Refactoring of the extend function in "using C libraries" to make it simpler.

Added tests to "Using Parallelism" part 4

Added tests to "Using Parallelism" part 3

Added tests to "Extension types" part 2

Added tests to "Extension types" part 3

Added tests to "Early binding for speed" part 2

Added tests to "Implementing the buffer protocol" part 2

Added tests to "Implementing the buffer protocol" part 3

Added tests to "Using Parallelism" part 2

Adding tests for "Unicode and passing strings" part 13

Adding tests for "Unicode and passing strings" part 14

Adding tests for "Unicode and passing strings" part 12

Adding tests for "Unicode and passing strings" part 11

Adding tests for "Unicode and passing strings" part 10

Adding tests for "Unicode and passing strings" part 9

Adding tests for "Unicode and passing strings" part 4

Adding tests for "Unicode and passing strings" part 7