Fix `VEC_SPLIT_OFF()`
Fix `VEC_SPLIT_OFF()`

Explain `VEC_IS_EMPTY()`.
No more `void` functions: return a `bool` (either a
 success value or `true`, so they can be used in logical
 expressions).
Mark some of the functions as `inline`:
 * `VEC_AS_MUT_SLICE()`
 * `VEC_AS_SLICE()`
 * `VEC_CAPACITY()`
 * `VEC_FROM_RAW_PARTS()`
 * `VEC_GET_NTH()`
 * `VEC_IS_EMPTY()`
 * `VEC_ITERING()`
 * `VEC_ITER_END()`
 * `VEC_ITER_IDX()`
 * `VEC_LEN()`
 * `VEC_NEW()`
 * `VEC_SET_LEN()`
 * `VEC_SET_NTH()`
 * `VEC_WITH_CAPACITY()`
 * `_VEC_INCREASE_CAPACITY()`
Make a dependencies list, with every symbol from each header.

SiIky/c-utils