documentation: core-api: rearrange a few kernel-api chapters and sections

Rearrange some kernel-api chapters and sections to group them
together better.

- move Bit Operations from Basic C Library Functions to Basic
  Kernel Library Functions (now adjacent to Bitmap Operations since
  they are not typical C library functions)

- move Sorting from Math Functions to Basic Kernel Library Functions
  since sort functions are more Basic than Math Functions

- move Text Searching from Math Functions to Basic Kernel Library
  Functions (keep Sorting and Searching close to each other)

- combine CRC and Math functions together into the (newly named)
  CRC and Math Functions chapter

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>