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 codebase more consistent. Including speech.h is not necessary anymore in some files. Includes have been removed.

synthesize.h now contains the definitions STRESS_IS_... that should be used with code related to syllable stress.

Note that isBreak and other defines were renumbered so that stress definitions could have values 0-6.

Possible TODOs:
1. Unify with terms used with phonemes, i.e. keywords like isDiminished in compiledata.c and stress_type in phsource/phonemes
2. Add functionality and documentation about STRESS_IS_PRIORITY and STRESS_IS_EMPHASIZED

Clang static analysis reports these as 'Result of operation is
garbage or undefined' errors.

Clang static analysis reports an 'Assigned value is garbage or
undefined' for this. If no points were read from the file, the
`n_points` variable is 0 and `env_x[n_points-1]` performs an
out-of-bounds access.

Clang static analysis reports an 'Assigned value is garbage or
undefined' error. This happens if the default_tune data has not
been copied in the kTUNE case.

Extending this further, it does not make sense to save the tune
data if the tune was not found, or if the tune already exists.