So they are protected from overflows, and shared between processes

This adds a corresponding header file for mnemonics.c that was introduced in d5d93be.

Going through files in src/libespeak-ng/, include-what-you-use removed a
few unnecessary includes and included explanations on why a certain
header should be included. This makes tracking globals and dependencies easier.

Running the codebase through IWYU should be repeated after each major
code restIncludes to standard c library weren't checked to avoid
breaking builds with other platforms.

See https://github.com/include-what-you-use/include-what-you-use

This makes the espeak-ng.c and speak-ng.c source code easier to
read and maintain. It also addresses bugs in command-line argument
parsing with the espeak implementation, for example it treats
`--compile-phonemes` as `--compile` due to an argument matching bug.