docs: Give the string input conversion function in the string tutorial a better name and clarify some comments.

Adding tests for "Unicode and passing strings" part 5

Adding tests for "Unicode and passing strings" part 3

Adding tests for "Unicode and passing strings" part 2

Adding tests for "pure python mode" part 11

Adding tests for "pure python mode" part 10

Adding tests for "pure python mode" part 9

Adding tests for "pure python mode" part 8

Adding tests for "Basic tutorial"

Added tests to "Using Parallelism" part 1

Added tests to "Memoryviews" part 1

Added tests to "Fused types (templates)" part 1

Added tests to "Interfacing with external C code" part 1

Added tests to "Implementing the buffer protocol" part 1

See #2362.

Enhance "wrapping C libraries".

Added tests to "Early binding for speed" part 1

Extended the examples of string.rst and put them in the examples directory for testing.

Added tests to "Extension types" part 1

Move a comment from the documentation into the example that it refers to, in order to avoid inconsistencies (e.g. line number references) and also to make it visible to those who only look at the file.
See #2353.

Added tests for "memory allocation" Part 1