Several code quality improvements

[eraxillan]

First of all, thanks for such tiny and useful library! My changes are:

1. CMake build script mods: CMake required version increased, C99 standard support requirement added
2. All possible compiler warnings were enabled (included warnings-as-errors), all current warnings were fixed
3. Address Sanitizer was enabled for debug builds
4. Code was wrapped to extern C to allow C++ interop
5. vector_size function was added to increase encapsulation